RAID = Redundant Array of Inexpensiive (now Independent) Disks. Simply stated, multiple hard drives running as a single drive when viewed by the operating system. Redundancy is the name of the game. If one drive in the array fails, there is no loss of data and the computer can continue to run normally. When the bad drive is replaced, the RAID controller will rebuild the lost data on that drive by using the remaining drives as a data source.
RAID 0 is not a true raid because it offers no redundancy at all. In this case, if one drive fails, all data on the array is lost.
RAID is not a substitute for backups as a virus or accidental file deletion will effect a RAID array the same way it effects a single drive.