For a good definition of RAID, see the following site.
http://www.webopedia.com/TERM/R/RAID.htmlBasically, it allows you to lose a hard drive and not lose any data.
You can also do a stripe raid which allows you to 'combine' many hard drives and use them as if they were one. A stripe raid does not reduce performence.
Combinations of the two are also available (Redundency and stripe).
A redundency RAID does reduce performence, unless your RAID controller/card has an onbaord parity chip that offloads parity operations from the CPU. RAID controllers integrated on motherboards generally do not have a seperate parity chip and use the CPU to perform RAID operations.
Generally, RAID with redundency is only used for servers or if you have highly valuable data.
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