If you've never worked with large systems with a bunch of disks, you probably don't have a lot of experience with RAID. RAID stands for Redundant Array of Inexpensive Disks (or if you ask some people, the I stands for Independent).
SearchSMBStorage.com has a good article on how the most common RAID levels work.
For a much more in-depth discussion (and visualizations of the way data gets written to where), check out this article from pcguide.com
It all boils down to how important the information is, how much disk space you need, and how many drives you have at your disposal.