RAID 0+1 is striping and then mirroring, so you need a minimum of 4 drives. The data is striped onto two drives and then mirrored onto another two. RAID 5 is also known as parity and requires a minimum of 3 drives. Data is written across the three drives and parity information is put onto the drives, so in the case that you lose one drive, you can still recover your data. Sorry, I can't explain it very well. Check
this out for a good explanation of all the different RAID levels.