Basically there are two main kinds of RAID
RAID 0 is when you take two hard drives that are exactly the same (size, manufacturer, model number, etc...) and combine them to be 1 big drive
IE. two 30 GB drives would show up at one 60 GB drive in RAID 0. You get improved performance because the RAID Controller Writes half the data to each drive.
RAID 1 is when you take two drives, and it writes whatever you save twice, once on the main drive, and 2nd on the 2nd drive. Used for backup reasons.