RAID, which is short for Redundant Array of Independent Disks, is a software or hardware storage virtualization technology that permits a system to use a number of hard drives as one single logical unit. In other words, all drives are used as one and the data on all of them is identical. This kind of a setup has 2 key advantages over using a single drive to keep data - the first is redundancy, so if one drive doesn't work, the data will be accessed from the others, and the second is better performance since the input/output, or reading/writing operations will be spread among several drives. There're different RAID types based on how many drives are used, if reading and writing are both handled from all of the drives concurrently, whether data is written in blocks on one drive after another or is mirrored between drives in the same time, and many others. Depending on the exact setup, the fault tolerance and the performance vary.