there are different types of RAID
The most common are 0, 1, 5, and 0+1 (10)
RAID 0 Splits data 2 (or more) ways to a number of identical disks for a huge increase in performance. If one disk fails, the whole array fails and data is unretrieveable
RAID 1 Copys the data to two disks, basically backup without you ever thinking about it, if one drive fails you've got the other disk with the same data on it.
Im not sure about RAID 5 lol google it
And RAID 10 is basically 1 + 0, an array of disks with another array of disks backing them up.
Hope that helped, best to check google or wikipedia.