RAID Levels:

0: Striping. Stores parts of data in each drive in the array to speed up
access time. Does not increase reliability at all.

1: Mirroring. All drives are identical, so that if one fails, the other(s)
will still have a complete copy of all data.

3: Striping plus parity drive. Works like RAID 0, except that an additional
drive is used to store parity info, resulting in data recoverability. (Thus,
at least 3 drives are required to use RAID 3.) RAID 3 is both fast like RAID
0, and reliable like RAID 1.

5: Striping plus multiple parity drives. Works like RAID 3, except that RAID
3 uses only a single parity drive, while RAID 5 uses all of the drives to
store parity data as well, resulting in better data recoverability, since
under RAID 3 you're screwed if the parity drive fails. RAID 5 is slower than
RAID 3, however.

0+1: Striping plus mirroring. Stripes across multiple drives, and uses a
mirror backup for each drive. Thus, at least 4 drives are required to use
RAID 0+1.