(from The Basics of Brewing by Scott Birdwell ©)
Many people know that beer is made from water, yeast, hops, and malt, but, many do not know what "malt" really is! Malt, in the general sense of the word, is a form of cereal grain (usually barley) that has been steeped in water for a while and allowed to sprout for a number of days. Then it is time to dry the grains as they will not keep if wet.  They are either put into a kiln or a roaster to dry.  The temperature(s) of the kiln or roaster and the amount of time in that vessel will determine how dark the grains become and what kind of flavors are being produced.  The decision by the malster between using a kiln versus a roaster also has an immense impact on flavor, as well.
What is happening is that when grain is malted, we are, more-or-less, "fooling" the grain seed into believing that it is springtime and it the right time to start growing. This sprouting process establishes enzymes within the grain that we will use later on in the brewing ("mashing") stage. We know that the desirable processes are taking place inside the grain seed by measuring the length of the sprout. It is these enzymes that we need in order to convert the starch in the grain into a simple sugar that the yeast can eat and convert into alcohol and carbon dioxide. The kilning process not only helps remove the moisture from the grain, thus rendering it in a much more stable state for storage purposes, but it also allows the maltster to select different levels roastiness and color. This is not unlike "Juan Valdez" and his Colombian coffee. Starting with "green" coffee beans, Juan can elect to roast them to a broad range of color, flavor, and roastiness. From the same green coffee beans, you can get a light roast coffee (a la Folgers) all the way to a virtually "burnt" flavor (as in espresso). With raw malted barley grains, we, too, have many options regarding its roastiness. We can produce a very pale yellow liquid (a la Budweiser) all the way to an opaquely black liquid (e.g. Guinness).
It is all a matter of:
  1. Just how hot is that kiln or roaster?,
  2. How long do I leave the grains in there?,
  3. What percentage of the grains should be pale in color to what percentage should be dark in color to achieve the desired color and flavor?
Grains from different regions in the world can produce markedly different end products, even when treated similarly by malting companies. But, broadly speaking, we can still separate the grains into two general:
  1. Fermentable grains. These are generally relatively light in color and still contain starchy flour inside their husks. During the malting stage, the barley "plant" (sprout) has produced much enzyme but most of the starch still remains unchanged. It is then kilned at relatively low temperatures. These grains need to be converted into simple sugars that the yeast can digest. This process is called "mashing" and good control of temperatures (ca.150º-160º F) during this steeping period is a necessity to achieve the desired effect.
  2. Non-Fermentable grains. These grains have been kilned at such high temperatures, that their enzymes have been destroyed and theirs starches converted and caramelized, thus rendering them "inedible" to most beer yeasts. For this reason, these grains do not have to be handled with the same care and attention by most homebrewers that the fermentable grains require. We are simply not concerned about the state of their enzymes and starches.
On most grains below, we will refer to º (degrees) lovibond. This is simply a measure of the color of the various grains. For example, a pound of crystal malt with a lovibond rating of 40º will impart just as much color as two pounds of crystal malt that is rated at 20º. Conversely, you would only need a tenth of pound of Black Patent Malt at 400º to impart the same amount of color as that pound of Crystal 40º. This information can be very helpful when trying to achieve the ideal color on a recipe, but does not tell you much about the flavors that the various grains impart. Three amber beers, one colored with Munich malt, one with crystal malt, and one with Black Patent Malt will taste radically different even if hops, yeast, temperatures, etc. are all the same.
Many books and recipes will refer to "two-row" or "six-row" pale malts. This is actually a reference to the way the raw barley grows on the stalk. What it means to the brewer is that six-row malt, having smaller kernels, has a greater percentage of husks and enzymes than two-row. This fact makes six-row the ideal choice for beer that are made up of relatively large percentages of adjuncts (e.g. American pilsners and possibly wheat beers). The additional enzyme helps convert the non-malted adjuncts and the extra husks are beneficial for lautering ("filtering") the "huskless" adjuncts and wheat malt. Unfortunately, the higher percentage of husks can also produce dry, astringent off-flavors in all-malt beers, so two-row varieties are generally preferred for any all-malt beers. An added benefit is that two-row varieties also tend to give 5 - 10% higher yields per pound because of the higher starch to husk ratio.
These grains should be "mashed" (steeped at about 155ºF) and "sparged" (rinsed with water at approximately 168ºF) at controlled temperatures prior to the boil:
PALE MALT - The lightest roast of barley malt, these grains are kilned at just hot enough temperatures to drive out moisture without damaging the enzymes within the kernels. Pale malts provide the majority of fermentable materials for most beers (even stouts! ). Virtually all malt extracts whether syrup or dried begin primarily from pale malts. Lesser amounts of darker grains can be added to the mash to produce amber and dark malt extracts. Actually, pale malt is a somewhat broad term and these grains can be more accurately subdivided into the following varieties:
--LAGER (OR PILSNER) MALT (1 1/2º - 2º lovibond) - Absolute lightest roast of the pale malts, ideal for light colored beers, such as pilsners and American light lagers.
--PALE ALE MALT (3º - 4º lovibond) - Slightly darker in color, ideal base for amber to dark colored ales. The darker roast adds a little character to the flavor profile. Beers produced exclusively from pale ale malt will tend to be gold colored.
--MILD ALE MALT (4º - 6º lovibond) - A roast darker than pale, mild ale malts can be used as a base for brown ales, milds, porters and stouts.
MUNICH MALT (Light & Dark) (5º - 15º lovibond) - An interesting grain in that Munich malt contributes some amber color and residual sweetness and yet is still very much a fermentable grain. It should be mashed, generally with a higher percentage of lager malt. Ideal for Octoberfests, Munich Dunkels & Helles, Bocks and other non-pilsner Germanic style beers.
VIENNA MALT (5º - 8º lovibond) - Another fermentable grain similar to Munich malt, but slightly lighter in color. Use similarly.
BISCUIT MALT (30º lovibond) - A marginally fermentable grain that should be used in place of "toasted" malt in many recipes. Biscuit malt produces a very pronounced "toasty" finish in the beer.
AROMATIC MALT (25º lovibond) - A mildly kilned malt that will add a strong, sweet malt aroma & deep color when used as a specialty malt. 
AMBER & BROWN MALT - Roasts of fermentable grains that, until recently, had virtually disappeared from modern commercial malting and brewing. Porters were formerly brewed exclusively from brown malt before it was discovered that one could substitute substantially smaller quantities of pale and add small amounts of highly roasted grains and still get comparable results for lower costs. Amber malt can be replaced with pale and crystal malts, again, with considerable savings in costs.  In recent years, we've seen a resurgence in the use of these grains.  Amber Malt imparts a strong biscuity, toasty sweetness to the finish of the impart and imparts amber & copper hues to the beer. Brown Malt contributes strong, dark-toasted grain flavor, slightly nutty with a hint of bitter chocolate character.  Great in Porters!
Honey Malt (25° lovibond) - With intense honey-like malt sweetness, Honey Malt has been patterned after German Bruhmalt and is well suited for use in brown ale and strong ale, among others.
Mashing is not required for flavor and color (although still not a bad idea!), simply steep like tea and give a quick rinse with hot water. This is sometimes referred to as a "mini-mash.")
CRYSTAL MALT (10º - 120º lovibond) - This is the most popular specialty grain used by homebrewers. We stock around different grains that fall under this umbrella.  Unfortunately, it is a rather broad term and can embrace a wide range of roasts (as you can see from the lovibond ratings! ). Crystal malt is taken "green" or wet from the sprouting vessel and is first gently dried for a few minutes at temperatures approaching boiling. The starch in the grain is converted into sugars and the interior of the grain liquefies. A further boost in temperature caramelizes these sugars, thus rendering them unfermentable. Upon cooling, the interior sets to a hard crystal. As little, if any, starches or enzymes can survive this treatment, there is no need to "mash" these grains for them to be useful. In fact, this unfermentable nature of these grains is the main reason to use crystal malt in brewing: we wish to increase the residual sweetness in the finished beverage. If the sugars were fermentable, they would simply be gobbled up by the brewing yeasts and converted into alcohol and carbon dioxide. By the way, this caramelization process also explains why many maltsters refer to crystal malt as "CARAMEL" malt. Continental maltsters may also refer to "CARA-Malt" (e.g. "CARA-PILS" (light crystal malt), "CARA-VIENNE" (medium-light crystal) and "CARA-MUNICH" (medium to dark crystal). The difference between light and dark crystal malts, whatever their names, is rather remarkable. Light crystal (10º - 20º lovibond) will add a noticeable amount of residual sweetness and mouthfeel, while contributing only modest amounts of color and that "caramel" finish to the flavor. The darker crystal malts add more color (amber to light brown), aroma and that caramel, "dark beer" flavor to your brew. For a real treat, try our Simpson's Double Roasted Crystal Malt.  This stuff is amazing!  Sweet, nutty, caramel-y, and adds a fantastic complexity to the finish of any amber to dark beer!  All of the crystal malts will enhance the head retention of your favorite grain beverage.
CARA-PILS/DEXTRINE (3º - 10º lovibond) - A roast of malt just under the lighter crystal malts, this grain is used as a "beer body builder" by enhancing the smooth finish in the beer and adding mouthfeel and body. This grain adds no appreciable amount of caramel flavor to the aftertaste. When a malt ceases to be "cara-pils" and becomes "light crystal" malt is a rather gray area, as both are used for similar purposes.
SPECIAL "B" MALT (175º - 200º lovibond) - A light brown roast, Special "B" serves as the "missing link" of malt, filling the large gap between the dark crystal malts (100º - 120º lovibond) and chocolate malt (350º - 400º lovibond). Special "B" can contribute a great deal of color to those brown-colored beers without the characteristic "toasty" finish found in beers colored with chocolate malt. Special "B" will add some (not a lot!) of sweetness to the finish. It is also a popular choice for brewers wishing to produce so-called "red ales" by incorporating small amounts (ca. 1/4 lb. per 5 gallons) in a "mini-mash" prior to the boil.
CHOCOLATE MALT (350º - 400º lovibond) - This grain is named more for its color than its flavor (although when combined with generous amounts of crystal, it can live up to its name!). Chocolate malt is a highly roasted grain, produced by loading pale malt into a roasting drum similar to a coffee roaster. The temperature is gradually increased until the grain just begins to carbonize. Ideal for porters and stouts (use 1/4 to 1 lb. per 5 gallons).
BLACK PATENT MALT (500º - 600º lovibond) - Similar to chocolate malt, but taken one step further, black patent malt is the "espresso" roast of barley malt. Use sparingly! (Sorry, Charlie P! ) A little of this can go a long way! Use 1/4 to 1 lb. per 5 gallons for your darkest, meanest stouts! The use of more may result in a beverage that more resembles an ashtray than a beer!
ROAST UNMALTED BARLEY (300º - 600º lovibond) - Roast barley can range in "roastiness" from chocolate malt to black patent malt. It is merely the unmalted version of these grains. A real "must" for your best stouts, roast barley seems to be slightly less harsh than its black patent cousin. Use in similar amounts.
WHEAT MALT (2º lovibond) - While technically not an adjunct, this a "wheat" version of pale malt. Therefore, this grain must be crushed and mashed to obtain any appreciable amount of yield and flavor. However, wheat, unlike barley, is a difficult grain to malt (no protective husk). When crushed, wheat malt virtually turns into flour. It is for this reason we recommend that you blend it (up to 65% wheat) with pale malt so as to have an adequate amount of grain husk to help separate out the "goods" from the spent grist. Without the help of the barley malt husks, an all-wheat malt mash would more closely resemble paste than sweet wort. It would be virtually impossible to sparge, as there would be no effective filter bed to help separate the "goods" from the spent grains. In addition, contains much higher percentages of nitrogenous proteins which can cause the beer to haze up when chilled. Small amounts of wheat malt can be incorporated into many beer styles to enhance head retention.
RYE MALT (3 - 4º lovibond) - Spicy, biscuity flavors.  Can be use for up to 60%+ of the grain bill.  The use of rice hulls is strongly recommended if using more than 5%.  Used often as you would wheat malt, but with spicy pumpernickel flavor.
Please note we now stock different roasts of wheat and rye malts.  These range in color from 35º lovibond Cara-Wheat (crystal wheat) to Chocolate Rye Malt (400º lovibond) and Midnight Wheat (550º lovibond - a sort of wheat version of Black Patent Malt). 
FLAKED BARLEY - Flaked adjuncts, such as flaked barley, are simply unmalted grains that have been fed through heated rollers which gelatinize their starches, therefore by-passing the need to pre-boil these grains prior to mashing. Flaked barley can impart a delightfully smooth grainy finish to the beer and enhance head retention. Typical use is 1/4 to 1 lb. per five gallons. Exceeding these amounts may lead to haze problems.
FLAKED MAIZE - Lends an attractive, sweet "corn-on-the-cob" flavor to some styles of beer. Flaked maize, when used in small amounts (1/4 - 1/2 lb. per five gallons) can add an interesting complexity to many styles of beer, including British bitters. When used in larger percentages (20 - 40%) it can produce a superior quality American style pilsner than the more commonly used, but cheaper, corn grits and has the added benefit of not requiring the gelatinizing "pre-boil" that the grits need.
FLAKED RICE - The use if rice adjuncts seem to produce a more "neutral" flavor than corn adjuncts and for this reason some American pilsner brewers prefer them. Again, the flaked version of rice omits the need to "pre-boil" the starchy grain before mashing in, thus saving quite a bit of time and hassle (but at the expense of higher material costs).
FLAKED RYE, FLAKED OATS (OAT MEAL) & FLAKED WHEAT - Yet more grains that can be added directly to the mash without "pre-boiling." All can add that pleasant "grainy flavor and enhance head retention. Use 1/4 to 1 lb. per five gallon recipe.