RAID 0 is mirror copy and requires 2 disks minimum. They contain the exact same information so if one fails, you can roll right over to the other without loss of data. You can replace the failed disk and it will rebuild the copy.
RAID 1 is used for performance with 2 disks minimum. It stripes the data across the drives, increasing read and write time. Downside is if one disk fails, you lose everything.
RAID 5 requires 3 disks minimum. It stripes the data across the three drives but leaves data for each drive on other drives, so if one drive fails, you can replace it and it will rebuild the data for the array.
RAID 10 (also known as RAID 0+1) is a combination of RAID 0 and 1 and provides the benefits of a RAID0 backup with the performance of RAID 1.
That's a basic outline. You can see more here:
http://en.wikipedia.org/wiki/Standard_RAID_levels