RAID is an acronym for Redundant Array of Inexpensive (or Independent) Disks. A RAID array is a collection of drives which collectively act as a single storage system, which can tolerate the failure of a drive without losing data, and which can operate independently of each other.
Various RAID Classes
RAID 0 Striping
RAID 1 Mirroring
RAID 0+1 Striped Set Mirrored
RAID 2 ECC
RAID 3
RAID 4
RAID 5
RAID 6
RAID 7 Proprietary
RAID 10
RAID 1E
RAID 50 RAID 05 + Mirror
RAID 53