RAID is Redundant Array of Inexpensive Disks. Raid is a way of making sure data is available constantly and is used on severs.
Although it is similar to backup it is different and you should back up all computers including those using RAID. Raid keeps data available if one disk fails but if two or more disks fail or computer is destroyed RAID is no help. you need a backup to recover from both of those disasters.
Raid splits the data on several disk and also writes a file containing information that tells how to rebuild if a disk is lost. Using RAID requires 2 or more disks.
In its simplest form called RAID 1 or mirroring you have two disks and everything is written to both disks. With other levels of RAID (most common are 3, 5, 10) you actually stripe the data. Part of each file is written on one disk, p[art on another and the information tat can be combined with information on surviving disks to rebuild is call parity.
Using RAID not only allows a system to keep on working when one disk fails but also with RAID 2 and above it speed up the access of the data. Since it is written or read from several disks the computer is able to be reading or writing several parts at one time instead of just being able to read or write to one disk.
RAID costs more. If you are using RAID 3 or 5 you need at least 3 disks to store the information you could store on one disk without RAID
Thanks to Laura for the question..