There are various types of RAID arrays but it really boils down to 2 options RAID for speed or RAID for protection of data.
Option one accesses the disks alternately so the lag time created by getting the disks attention, sending control signals etc overlaps the read write time on the other disk(s). This is used for speed.
Option two uses various methods to write your data to multiple disks so that if one disk crashes, the data can be rebuilt from what's left on the still working disks.
There are a variety of types which mix'n'match the above. I can't remember off the top of my balding head what is called what but I'm sure a Google will answer that question.