RAID

Definition by Web Hosting Ratings:

Redundant Array of Independent (or Inexpensive) Disks.  Type of disk, often used on servers, where several physical disks are combined into an array for better speed and fault tolerance.
Level 0 implements data striping where file blocks are written to separate drives.  Does not provide fault tolerance, because failure of one drive will result in data loss.
Level 1 implements data mirroring.  Data is duplicated on two drives either through software or hardware.  Provides faster read performance than a single drive.
Level 2 - not used in practice.  Data is split at bit level at written to multiple drives.
Level 3 - requires at least 3 drives.  Data block is striped at byte level across drives and error correction codes (parity info) is recorder on another drive.  Provides fault tolerance but slower writing performance.
Level 4 - Similar to Level 3 but provides faster performance because it uses blocks for striping.
Level 5 - Similar to Level 4 but improves performance but also striping parity info across multiple drives.
Level 6 - Similar to Level 5 but also uses second parity scheme for better fault tolerance.
Level 7 - Proprietary RAID design by Storage Computer Corporation.  Faster than other levels because it uses multiple levels of cache and asynchronous I/O transfers.
In addition multiple RAID levels can be combined to improve performance or reliability.