>>1
It's basically a way to treat multiple hard drives as one. This can be used for speed (in the case of RAID 0), or, more commonly, for data safety. For instance, RAID 1 uses two hard drives and mirrors data between them. If one drive goes kaput, the other drive still has all the data on it. The more complex configurations use three or more hard drives.
You can also lessen the tradeoff by having RAIDs of RAIDs, which is often done in high-performance servers.