raid is used to improve performance or to provide fault tolerance. There are various RAID systems available.
RAID 0 (Stripe Set)-> min requirement is two disks. it splits up each file into 64kB Blocks, so when the things are read from disks they are read at same time but with improve performance. Raid 0 has no fault tollerance, that is if we lose one disk we lose all data. its not suitable if file size are really small. normally it is used for storing paging files.
RAID 1 (mirror set)-> two volume of same size. it stores exact copy of files on both disks. if we lose one disk all data is availbel on other disk. so the advantage here is fault tollerance. The downside is its very slow and expensive. There is a 50% ollerance)
RAID 5 (Parity Set)-> min requirement is three disks. similar to raid 0 it splits data. The idea is that if we lose one chunck, we can recover data by putting togather other chunks. Parity proides improve read performance, but write are slow. And it support single Fault Tollerance. The Overhead here is 1/n. So if we have 3 Disks then (1/3) 33%. Which means third of space will be used by parity, rest of it available for data. xp supports raid 0, 1. Server supports raid0, 1, 5..

Practicle Implement RAID DISKS:
for Mirror: comp mgt -> right click on volume & then select add mirror