What is RAID?

Alright, I know I'm an idiot but I still don't understand the concept well. What exactly is it and what can it be used for the common user?

>>7
I had a RAID 0 solution for a couple of years. My motherboard came with onboard RAID support and I had two identical drives. It was neat for the novelty for a while but after I realized it's more trouble that its worth what with MTBF/2, needing a driver disk to install windows, the virtually zero noticable performance increase in my machine, etc. I dumped it.