RAID (Redundant Array of Inexpensive/Individual Disks)
"It is a storage method that uses numerous devices in combination to store large amounts of data"
So if you have more than one hard drives, you can use it in RAID 0 (striping - no fault tolerance: if something goes wrong, goodbye to data, fastest, size is equal to how big the drives are)
or RAID 1 (mirroring - fault tolerance: stores same data on both drives, so if one drive fails, swap it out and you have the data still. Size is the size of one hard drive, because same data stored on both drives.)
or RAID 5 (striping with parity - fault tolerance, requires at least three drives. Two drives are used to store data, one is used to store parity, so if one fails, you can recover it from the parity. little Faster, but size is not as big as RAID 0, because you use the third drive for parity.)
or RAID 0+1 (mixture of both 0 and 1, requires at least four drives. two are used as RAID 0 while the other two are also used as RAID 0, but the second set is a back up of the first set.)
Hope this helps