RAID is an acronym for redundant array of inexpensive drives (or disks) also known as redundant array of independent drives (or disks) and is a type of storage configuration which uses two or more drives for fault tolerance, performance, throughput or capacity. In its most simple form RAID combines two or more hard disks to form a single logical storage unit. There are a number of different RAID levels helow is a beief description of the most common:-

RAID 0 - Striped: Data is spread across two or more disks providing increased performance but no fault tollerance. If one disk becomes unavailable the whole RAID array is lost.
RAID 1 - Mirrored: Data is duplicated across two or more hard disks therefore increasing the fault tollerance. If one disk goes down the data is available on the second disk.
RAID 5 - Striped with Distributed Parity: Distributed parity requires all but one drive to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure.