Craig Farrell (3/24/2011)
Cat, it's for both, and same purpose. In a RAID 5, for each section of disk (I forget the break out) in say a 4 disk array, 3 have data, one has a parity to make sure the other three stay intact. Because of that parity, you can assume the data on the 'lost' disk while it's replaced, and rebuild it, due to reverse construction of the bit/bytes. Same concept, expanded further.
Lose two drives and it's game over though, as most folks are well aware.
I believe that the data is actually stored on 3 disks...2 have the actual data and the 3rd has the exclusive OR of the other 2 disks. Using this, if you loose any one of the 3 disks that store the data you are after, the other 2 have enough information to give to retrieve the data. This basic pattern is used regardless of the number of disks in the array. When there are more than 3 disks, each chunk of data is still stored on 3 of the disks. The controller manages the allocation of disk space to ensure that all disks on the array are utilised.
Steve - whilst you may not have heard of checksum (Vs parity) in relation to RAID 5 , try searching google for "RAID 5 checksum". I do agree that you can view this as a "parity" but I am one of them dinosaurs who have been around since the days of punch cards, paper tape and magnetic tape. Parity back then was more to do with detecting error.