[fallen1]

Scoot, RAID is a way of setting up multiple hard drives for increased data safety and/or access speed. There are several differrent RAID types, but the most common amoung home users are RAID 0, RAID 1, and RAID 0+1. I'll do sort of a general rundown on them for you.
RAID 0 is also called a striped array, and usually uses 2 or 3 hard drives. What it does is treat all the hard drives as if they were one drive and splits everything up between them for faster read/write times. So if you had 2 150Gig drives in RAID 0 your computer would see them as a single 300Gig drive and would split everything up so half of each file would get written on each drive. The upside of this is that stuff can be written and read faster since each drive only has to read half of a file. The downside is that if one of those drives goes bad you loose everything because half of every file went with it.
RAID 1 is called a mirror array, and uses 2 drives. It treats both drives as if they were one drive, but only sows the capacity of one of the drives because both have copies of everything. So if we used the same 2 drives from the RAID 0 example, it would show the 2 drives having 150Gigs of space. Everything on the computer gets copied to both drives, so if one dies there is a complete working copy of everything on the other drive.
Both RAID 0 and RAID 1 require that you use drives that are the same size. You can use different size drives, but they will both be seen by the computer as being the same size as the smalle of the 2. So if you have a pair of 150 Gig drives, you could have a RAID 0 with 300 Gig or a RAID 1 with 150 Gig. If you have a 200 Gig drive and a 150 Gig drive, both drives would be treated as 150 Gig for the same 300Gig RAID 0 or 150 Gig RAID 1 totals because the RAID controller needs matching drive sizes to work. You wouldn't be able to use the extra 50 Gigs on the 200 Gig drive because the RAID array wouldn't have a matching 50 Gig somewhere else to use for either the striping or mirroring.
A RAID 0+1 array uses 4 drives to do both mirroring and striping. so if you used 4 150Gig drives, your computer would see them as 1 300Gig drive. What it does is create 2 RAID 0 arrays that are clones of each other. 2 of the drives make one 300Gig RAID 0 and the other 2 make a 300Gig RAID 0 copy of them. This gives you the speed boost of RAID 0 with the protection of RAID 1.

I hope that all made sense and helps you out with figuring out how the different RAID set ups work. If you want some more info, there's a pretty good article about RAID on wikipedia.
[link=http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks]http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks[/link]