Just a way to combine multiple physical disks into one logical disk as seen by the operating system. This can (but not always) include fault tolerance to provide error checking and repairing.
Redundant Array of Inexpensive/Independant Discs. It is where you can connect two or more hard drives in RAID 0, for double the space, or RAID 1, for mirroring, for example.
Anywho, raid 0 rocks, and so does raid 5. I say if you have 2 sata hd's do raid, it'll be much healthier for your computer, especially if you have 3 sata hd's and can pull off raid 5.