RAID - Redundant Array of Independent Disks. A Storage technology to provide redundancy and performance, to reduce I/O bottlenecks involved in using multiple disks for large storage.
The basic idea of RAID was to combine multiple small, independent disk drives into an array of disk drives which yields performance exceeding that of a Single Large Expensive Drive (SLED). Additionally, this array of drives appears to the computer as a single logical storage unit or drive.
More at Wikipedia : http://en.wikipedia.org/wiki/RAID