There are different levels of RAID. RAID0 is commonly used by gamers and it is basically using 2 or more disks as one in order to achieve higher read/write speeds.

Mirroring one disk to the other is RAID1.

RAID5 (what I want) is the ability to take 3 drives (in my case) and use the capacity of two of them as one drive. The third drive-space stores parity information in case one of the other drives dies. If that were to happen, I could recover the data on the dead drive to a new drive and continue as if nothing happened. When we're talking about 100s of gigabytes of music that's a pain in the ass to rip, having this kind of security is desireable (IMHO).

There are probably 10 different RAID levels. 0, 1, and 5 are the most common though.