RAID stands for Redundant Array of Independent Disks and
it basically involves 
combining two or more drives together to improve the
performance and the fault tolerance. 
Combining two or more drives together also offers improved
reliability and larger data volume sizes.A RAID distributes
the data across several disks and the operating system
considers this array as a single disk. 
Using Multiple Hard Drives for Performance and Reliability.

      RAID 0+1
      RAID 1+0
      RAID 3+0
      RAID 0+3
      RAID 10+0
      RAID 5+0
      RAID 6+0

For More details  :  http://newadmins.blogspot.com/