-mmmdreg
RAID is an acronym for Redundant Array of Independant Drives. And as the name suggests, it requires more than one hard drive, and prefers all the drives to be the same size.
The most obvious effect of a RAID'ed set of drives is that, to the computer, they look and therefore act like one hard drive, instead of several.
There are levels of RAID, each doing different things. Level zero (0), is also called "striping", and all it does is make the drive look like one.
However, when you get up to level 3 or higher, the drives actually begin to parse the datas you are saving to them into pieces and distribute the pieces evenly across some of the drives - and others in the array back up (Redundant) the first set. This way when one of the drives fail, you don't lose anything.