Redundant Array of Independent (mistakenly called Inexpensive sometimes) Disks is a scheme that allows multiple hard disks to be used to create a single unit with features better than what would be attained if they were not a unit. A “RAID controller” contains the circuitry to manage the disks as a single unit. There are different RAID schemes. The most popular ones are RAID 0, 1, 0+1 and 5.
RAID 0 allows Striping, a scheme where alternate blocks of data is written to different disks. This results in a direct doubling of the write / read speeds. The flip side is that if one of the disks fails, the entire data becomes inaccessible.
RAID 1 offers data redundancy by mirroring the data. In this scheme, the same data is written to both disks. While there is no improvement in performance, from the security standpoint, failure in one disk will not result in downtime, since the same data is mirrored on the other disk(s). Both the above-mentioned schemes require at least two hard disks to function.
RAID 0 + 1 brings together the benefits of both the schemes, and obviously requires a minimum of four hard disks to function. RAID 5 allows striping, but also stores the parity information of every operation, which can be used to recover data in case of failure. This scheme requires a minimum of three drives.