Ah, just like using multiple disks at one time in case one fails (redundancy), although not always the case (RAID 0 is just striping with no parity). And also to pull some more speed when reading from the drives (two or three drives transfer data faster than one). But it all depends, For instance the disk mirroring (RAID 1) makes duplicate drives, where on reading either and/or both the disks can be used however when writing to the mirrored array, both disks must each be written to, so the gain in performance is really on the reading, not the writing. However a drive can crap out on you and it can just be replaced - hot swappable usually. Whereas something like RAID 0 which is striping has files spread across both drives where say, a file is read and parts 1,3,5 and 7 come from Disk 1 - parts 2,4,6 and 8 come from Disk2 and so on to make the total file. In this way both disk drive heads are working at the same time to achieve faster times. It would be the same when writing to it too, faster because data needs to be written only once across both disks, unlike the mirroring. However with a striped set, if a drive fails, you are boned.
Then there is stuff like RAID 5 that require at least three drives. Imagine that one disk contains is one set area the data that is the number 5 - the next disk in the same set area is 3 and the final disk has the set area a result of the other two, like 5 + 3 = 8 which is striping with parity, where any given disk can fail and the system can rebuild that disk when replaced based on the remaining data from the other two, so imagine:
5+3=8
Drive a fails x+3=8
Drive b fails 5+x=8
Drive c fails 5+3=x
If any of that badly written mess makes sense?
Thats already a mess though - Cause I couldnt be bothered typing so much, here is a link.
http://www.adaptec.com/worldwide/product/markeditorial.html?prodkey=quick_explanation_of_raid