Raid 6 is an improvement on RAID 5 that allows for failure of any two drives with data remaining in tact as opposed to single drive failure in RAID 5. It does this by using two full drives for parity.  Data is striped across all drives with redundancy on two of the drives.  Parity sectors are rotated through all drives so all drives are used evenly throughout the array.

Because of the added parity, there is twice as much as much waste as there is with RAID 5, but the added security of a second parity drive allows for significantly more data security — data is still as secure as in a RAID 5 while rebuilding the array from a single drive failure in RAID 6.