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