RAID stands for Redundant Array of Independent Disks. Basically, for people that work with servers, it's uber great because it actively backs up new data on the fly. There are about 10? different configurations, with RAIDs 0, 1, and 5 configs being the most popular. RAID 0 is generally most used for gaming and hard core machines, but has the risk of losing a MUCHO amount of data if one of the drives fails.
Basically with RAID 0, you have two 500 gig(or whatever equal sized)drives with about 100 megabits of read speed and when two come together, it turns into a terabyte with 200 megabits of read speed. Essentially, you can think of it as a dual core processor for storage, two processors work together in one and gets more work done. A RAID has two or more drives working as one with faster read speeds.
RAID 5 is like 0 but it has redundancy, meaning two hard drives actively back up the other two drives. It's great since if one drive dies, just throw a new one of equal size and the array will rebuild itself over a period of time, depending on how much data you have.
It's a bit much, but oh well!
Then there's Solid State Drives, but those are bank breakers at this current spectrum of time....