This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.
The primary functions of a disk array is to increase data availability, to increase total storage capacity, and to privide performance flexibility by selectively spreading data over multiple spindles. Data Protection - As the number of disks on a system increases, the likelyhood of one failing increases. Thus, a disk array should be immune from a single disk drive crash. Disk mirroring (keeping an exact copy of a one disk on another) is the simplest, but requires twice the disk capacity (and associated cost). Encoding schemes can be used to reduce the redundancy required to lower ratios. Storage Capacity is increased by placing many smaller form factor (5.25 and 3.5-inch) drives onto an intelligent controller which makes all the drives appear as one drive to the computer system. Performance can be increased by spreading data over spindles and performing operations in parallel which allows multiple drives to be working on a single transfer request.