RAID allows two or more disk drives to be seen by the operating system as one physical drive. This allows for fault tolerance on your computer so that if one drive fails, the data is securely still available on the other drive(s).
There are different levels of RAID. Some are described below:
RAID 0 - RAID Level 0 is not redundant, hence does not truly fit the "RAID" acronym. This method splits the data over numerous drives but is seen as one drive by the operating system. If one drive fails though, all data becomes inaccessible. This level is commonly referred to as striping.
RAID 1 - This method uses two drives. One drive is the active drive and the other drive is used to store the same information as the active drive, in other words they mirror each other. This method is also known as mirroring.
Raid Level 5 - This method is used with 3 or more drives. This method allows for one drive to fail yet the computer continue to run.