Techgage explains what RAID is all about, exploring all the options and discussing the possible dangers. You can read their guide over here.
RAID stands for "Redundant Array of Independent Disks." This brings up the first point - RAID is for redundancy; NOT A BACKUP! This is, above all, Rule 0 of RAID.
Say it with me, people: RAID is NOT a backup.
So if it's not a backup, what is it? A RAID array is a group of hard drives that are treated as if they are one congruous file system. That filesystem does one or both of the following: speeds up read/write access (which is called striping) by spreading a single file over multiple disks, and provides fault tolerance, which is the ability for one or more disks to fail without losing all of your stuff by writing the same information (or a checksum, called 'parity') to more than one disk (called mirroring).