RAID stands for Redundant Array of Inexpensive (or Independent) Disks. Motherboard specifications will list what RAID setups (the numbers that follow RAID) are compatible with it and RAID controllers can also be purchased. Setting up a RAID system can provide a couple of different benefits depending upon which configuration you use. If you are looking to increase performance, there is RAID 0 and if you want to back up your files there is RAID 1. There are other options, but tend to be more expensive and difficult to set up.
RAID 0, or striping, combines two drives and writes the data to both drives simultaneously, which increases the bandwidth and speeds up the process. This can be useful for gaming or giving your system a little bit more speed. The drawback is that if one drive fails, the whole setup fails and all the data is lost because the data has been split up into blocks and placed on separate drives.
RAID 1, or mirroring, uses two drives to mirror one on the other. The data is put on one drive and identically placed on the other drive. If one drive fails, then the other still has all of the files on it. This setup is particularly useful if you have a lot of important data or you have lots of media files that you are concerned about losing.