Redundant Array of Independent Drives (or Disks), also KNOWN as Redundant Array of Inexpensive Drives (or Disks) (RAID) is an IMPORTANT term for data storage SCHEMES that divide and/or REPLICATE data among multiple hard drives. They offer, depending on the scheme, increased data reliability and/or throughput.
RAID is a way of storing the same data in different drives(thus, redundantly) on multiple hard disks.