Here is a very high-leveled description. http://www.webopedia.com/TERM/R/RAID.html
Different RAID levels perform in different ways. It's basically a way to increase performance, data recovery, data redundancy and data integrity by implementing a level of RAID for 2 or more alike hard drives.
There are many different RAID levels. There's RAID 0, 1, 0+1, 2, 3, 4, 5, 6, 7, 10, 53. All of these levels require a different amount of drives and perform in very different ways.
I'll find another site that explains it a little better and post it here.