Importance of barley to brewing
Barley is the grain most closely associated with brewing, as well as one of the earliest crops domesticated in the Near East, facts which some have speculated may be related. The bulk of most modern grain bills consists of some sort of malted barley.
Barley is sometimes grown by adventurous homebrewer-gardeners, although it is much less common than home hop cultivation.
Types of barley
Two-row versus six-row barley
The barley plant comes in two types: "two-row" and "six-row", differentiated by the number of kernel rows in the barley head. There is also "four-row" barley, but for brewing purposes it is identical to six-row and "four-row" is not encountered as a brewing term.
Two-row barley is the older form of barley, closely related to wild barley. It is the traditional European brewing barley and used as a base malt in most traditional European beer styles.
Malted versus unmalted barley
Unlike many other brewing grains, barley is almost always used in its malted form, since malting develops the enzymes needed to convert barley's starch to sugar. Unmalted barley is sometimes used in brewing as well, especially in the form of roasted barley.
Types of barley available to brewers
See below for details on specific types of barley malt and other barley preparations available to home brewers.