RAID stands for “redundant array of independent disks," or sometimes “redundant array of inexpensive disks." RAID refers to a hardware arrangement that allows for the storage of data in multiple places on two or more disk drives used in combination. The use of RAID can enhance the performance of a system by allowing input and output procedures to overlap, and has the additional advantage of providing fault tolerance. Fault tolerance refers to the level of a system’s capability to respond to problems, such as hardware or software failure. RAID improves fault tolerance because it extends the range of time that a system can be expected to function reliably between failures, which is commonly referred to as the mean time between failures (MTBF).

There are at least nine established levels of RAID, each using different combinations of techniques and having their own benefits and disadvantages. To the operating system, which manages the application programs of a computer, RAID is perceived as a single hard disk. A defining element of RAID is the use of disk striping, which solves the problem of slow data transfers that limit performance in systems employing a single drive. Striping divides larger files into small pieces that are stored across multiple drives such that the files can be retrieved efficiently as the drives work in parallel.

There are two data redundancy methods used in RAID, mirroring and parity. Disk mirroring is a technique that is used in some RAID levels, particularly RAID Level 1 but also in other levels which incorporate Level 1. In disk mirroring, at least two drives are used to simultaneously store the same data, adding some level of data protection to the array. In the case that one of the mirrored drives fails, the system can automatically switch to using the functional disk without downtime or data loss. Though disk mirroring does not replace periodic back ups for critical data altogether, it is being increasingly implemented in sectors such as the financial industry as an alternative to the highly inefficient and basically antiquated tape based back ups. Parity refers to a technique which uses given data to compute an extra piece of data, and stores the multiple pieces of data across multiple drives. The premise is that if one piece of data is lost, it can be recreated using the remaining data. Parity provides a cost advantage over mirroring, as many disks are not required.