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 does just what it sounds like: 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 data loss has occurred. You would usually see this for system disks in uptime-critical servers.
There also exist more advanced modes, the most common of which is called RAID-5, and consists of 3 or more disks with data stripped (written sequentially), along with parity information, across the disks.
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, and should not ever be used for redundancy.
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.
