RAID stands for Reduntant Array of Inexpensive Disk's. I'm not gonna go into the history of it or the diff types at this time, but basically you can hook up different Hard Drives in a RAID config for increased performance, or for data securuty. Its a good idea to do because its relatively cheap and makes your comp a lot faster. Btw, why only 250 GB HD? would cost little to upgrade that too a 500GB or a 750GB.