If you need to ask you probably don't need one
RAID stands for Redundant Array of Inexpensive Disks - it's a way of increasing performance and/or reliability by making multiple disks appear as one to an operating system. As a general rule, this comes at the expense of requiring more disk space to make this happen - eg, you can shield yourself from a single disk failure by mirroring, but you need twice as much disk space to do this.
There's lots of info on the 'net - try Googling for "RAID controller" or similar.
Bruce
