Redundant Array of Independent Disks
Ideally they would be all the same drive. I've paid extra for sequential serial numbers.
There are different levels of RAID for different missions but the short version is Raid 0 = speed with no backup, Raid 1 is full backup but you lose half the drive space. RAID 5 allows the loss of one drive from a set of at least 3. There are more levels but there are entire sites dedicated to explaining it in depth. Google is your friend.