RAID stands for Redundant Array of Inexpensive(or Independant) Disks. A RAID array is a collection of drives which collectively act as a single storage system, which can tolerate the failure of a drive without losing data, and which can operate independently of each other. There are a few RAID levels. Most people are concerned with the first two(0 and 1):
-RAID Level 0 (Striping) is not redundant, hence does not truly fit the "RAID" acronym. In level 0, data is split across drives, resulting in higher data throughput. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in data loss.
-RAID Level 1 (mirroring) provides redundancy by duplicating all data from one drive on another drive. The performance of a level 1 array is only slightly better than a single drive, but if either drive fails, no data is lost. The cost per megabyte is high in this set-up, however your pretty much guaranteed of safe data.
So basically if you want a faster hard drive set-up, use level 0, and if you want safer set-up use level 1. You also need 2 identical hard drives (size and speed at least) to work in a RAID set-up. Maybe a RAID set-up isnt what your in need of??