Most malts are categorized as either base malts or specialty malts. In any beer recipe, the majority of the grain bill will consist of the base malt. These malts are generally very light in color, and provide the beer with the bulk of the fermentable sugars. Two-row pale malt, for example, is arguably the most frequently used base malt, found in a wide array of English and American ales, as well as some lagers. If your an extract brewer, malt extract is your “base malt”. Malt extract is simply a concentrated wort (that’s what we call unfermented beer) that’s been spray-dried or reduced to a thick syrup — just add water!
Specialty malts, unlike base malts, don’t contribute much in the way of fermentable sugars. They’re primarily used for adding the “character” to the beer’s malt complexity. Crystal malt (also called Caramel malt) is an extremely common malt used to impart sweetness, color, body and head retention to beer. Rated on the Levibond scale from 10L to 120L, darker crystal malts have been kilned longer, and provide not only richer and darker color, but also more pronounced flavor.
For more information about different malts, check out Briess’s website. It’s a great resource that explains the malting process, as well as describes the characteristics and uses of a number of commonly (and some not-so-commonly) used malts.