Tim is correct, RAID is not what you want. But I'll give you some info on it anyways in case you're curious. You might even like some of its abilities.
RAID stands for Redundant Array Of Inexpensive (or Interdependent) Disks
It's a method of hooking up multiple drives inside a computer so the computer sees it as one big drive, and gives you the ability to gain increased performance or fault tolerance.
The most common RAIDS these days are Stripes and mirrors. (RAID 0 and 1 respectively)
A RAID 0 Stripe, using 2, 1TB drives would be seen by the computer as a 2TB drive. It would also be marginally faster then a single 2TB drive. The downside of this is that if one of the two drives fail, you loose all the data on both drives, because the computer does not distinguish between the two physical disks.
A RAID 1 mirror would take 2, 1TB drives and show the OS a single 1TB drive. The second drive keeps an exact mirror of all the data on the first, This gives you fault tolerance as if one drive dies, all your data is still backed up. a RAID mirror is a very secure way of storing all your data.
There is also RAID 5, which uses 3 physical disks and each disk stores parity data for the other 2 drives. If one drive fails, you can replace it, and rebuild the data from what is on the other 2 good drives. It nets you a bit more storage space then a mirror does, and slightly better performance depending on what you're doing, but is mostly used in servers.
I keep all my data on a RAID 1 array in my computer. I don't worry about my hard drive failing and me losing all my data anymore. However, I still back up all my data. A RAID array will not protect your data from a lightning strike. A DVD backup will.