RAID is a common technique to provide _resiliency_ and _availability_ to a set of data and protect against one of the most common data loss scenarios: the failure of a disk.
The simplest type of RAID is a 'mirror', which keeps two or more copies of data on two or more different disks. If one disk fails, the second copy is still available and no availability or data loss has occurred. You would usually see this for system disks in uptime-critical servers.
More common for bulk data are more advanced modes like RAID-5 and RAID-6, which consists of 3-4 or more disks with data stripped (written sequentially), along with parity information, across all disks, and which can tolerate losing 1 or 2 (respectively) disks in the array before losing data.
It's worth noting that a pure 'stripe', also called RAID-0, is not really a RAID level - it *increases* the risk of data loss rather than decreasing it, since one disk failure destroys the whole array. It should never be used for redundancy or any critical data.
The [Wikipedia page for RAID](http://en.wikipedia.org/wiki/RAID) provides some helpful information about the history and benefits of the various RAID implementations.
