What is malt?
Short answer: malt is grain (usually barley) that is processed in order to convert grain starches to sugars. There are a vast number of malts that brewers can use, and there are two broad categories that various malts fall into: malts which can be steeped, and malts which need to be mashed. Specialty malts and crystal malts (you may see the latter referred to as caramel malts, which is simply a different term for the same thing) can be steeped. These have sugars and flavor compounds available as-is. Simply cracking them, and steeping them in hot water will impart their flavors to the beer. Other malts and grains are mostly made up of starches. These starches need to be converted to sugar by the brewer, which is where mashing and sparging comes in. If you're an extract brewer, you'll want to stick to the former category. As a new, extract brewer I found the variety and nomenclature extremely overwhelming. This is my attempt to clear some of that up.
Crystal malts are steepable and they're generally used to add sweetness and color to both extract and all-grain brews. They're usually named based on color (which can be very specific and scientific [based on the Lovibond color scale - e.g. Caramel 60 L], or pretty subjective [e.g. "So-and-so's Dark Crystal]). As a general rule, the lighter-colored crystal malts are more strictly 'sweet,' while darker crystal malts can add some roastiness or nuttiness in addition to sweetness. On the extreme light end, there are dextrin(e) malts. These malts also add dextrins, which contribute body and a thicker mouthfeel to beer. To confuse things, many maltsters have their own trademarked brand names for certain malts which can obscure what they really are. CaraFoam® , for example is Weyermann's dextrin malt. So CaraFoam®, Briess Carapils®, Caramel Pils, and Dextrin Malt are all different names for very similar malts. Basically, anything labeled crystal, caramel, or cara-something are crystal malts (with the exception of Weyermann's CaraFa® which we cover next).
Roasted Malts/Dark Malts
Roasted malts are any malts or grains that are roasted to a very high degree. Any very dark (say, more than 150 Lovibond) malt is considered a roasted malt. The three most common roasted malts are: black malt (sometimes called black patent malt), chocolate malt, and roasted barley. Also belonging to this group are Weyermann's range of CaraFa® malts, Kiln-coffee malt, and distaff cousins like de-bittered black malt and pale chocolate. Roasted malts can be steeped for extract brewing or mashed for all-grain, and add a lot of complexity and color in very low quantities. Some brewers get gun shy about roasted malts, but fear not. Roasted malts are delicious, provided you don't go completely overboard: 10% (or roughly one pound in an average-gravity 5 gallon batch) is about the most you would usually use. Stay below this amount and it's hard to go wrong. Go right, and roasty, bready, biscuity, coffee-y, dark chocolate, and a host of other flavors are at your disposal.
Base malts make up the majority of the grist in all-grain beer. This group includes pale malt, Pilsner malt, Vienna malt, Munich malt, Mild ale malt, and more; there are also non-barley base malts like wheat malt and rye malt (more on these below). The variety is frankly, astounding.
Base malts can be named based on the formation of corns on the barley stalk (2-row vs. 6-row), the barley variety (e.g., Maris Otter, Golden Promise, etc), or the region in which the barley was grown or malted (America, England, Moravia, etc).
American base malt is generally mild and fairly neutral; British malts tend to be maltier, bready, and biscuit-like. The European climate gives malts made from Continental barley a clean, "elegant" character. Pilsner malt has a soft, delicate maltiness that practically defines pale lagers. "High-kilned" (heated to a higher temperature at the end of the malting process) base are responsible for the dark, malty lagers of Europe and have also found a home in some ales because of their unique character. Munich and Vienna malts are the prime examples of high-kilned malts, although mild ale malt belongs to this category too. The darker color lends these malts a more toasty, malty flavor than you get from lighter base malts.
Adjuncts are unmalted, starchy ... things (normally understood to be a cereal grain, but homebrewers have been known to use things like pumpkin and potatoes, too). Adjuncts don't have sugars available like crystal malts, so they can't be steeped for extract brewing. They also don't have enzymes like malted grains, so they need to be mashed with base malt to extract their sugars. Examples of brewing adjuncts are flaked barley, flaked oats, maize, and torrified wheat, among others. Pumpkin and squash beers are not uncommon. Potatoes and rice have also been used in beer. Any starchy vegetable or grain can be used as an adjunct. These grains can add some characteristics to extract beer but they really need to be mashed to unlock their full potential.
There are also some malts which do not come from barley: oats, rye, and wheat can be malted. These malts are essentially processed like, and can be treated as, their barley malt cousins. Caramel wheat is similar to caramel barley malts and the same for non-barley wheat base malts, and so on. The one difference with these malts is in how they're crushed. If you crush your own malts, you will want to do some testing before you run a whole batch's worth of rye malt or oat malt through a mill. Wheat malt can be crushed at the same setting as barley malt.
"Other malts" also includes specialty malts which don't fall into the other categories of barley malts: things like biscuit malt, or aromatic malt. Frequently these malts are used in low quantities to contribute unique flavors. Fortunately, the names given by their maltsters are usually obvious as to what sort of flavor they contribute. Biscuit malt, for example, contributes a very biscuity flavor (sometimes described as 'saltine cracker' flavor), aromatic malt adds a very malty aroma and a deep malt flavor. There are some specialty malts which are less clearly named. Some of the toasted malts like Victory, Amber and Brown malts, and special roast are less obvious. Brown malt and amber malt are similarly toasted malts with brown being darker and more toasty and bready. Amber malt is lighter in color and has less of a pretzel-like flavor and more of a light bready flavor. Victory malt is another light one which sort of lies in between biscuit malt and amber malt with characteristics of both. Special roast is fairly unique and will impart a slightly darker, reddish color and has a fairly strong tangy, berry, and deep almost alcohol-like flavor. All of these can generally used in low quantities at 5% of the fermentables, or half a pound in a standard 5 gallon batch, or less.
So there you have it. This is a basic guide that I hope helps determine which sorts of malts are steepable or need to be mashed etc. If nothing else, experimentation is one of the joys and freedoms that brewing your own beer provides. Some of my best recipes have come from combining a new base malt with some crystal malts and/or specialty malts I've never used before.