you need to do some reading really:
http://en.wikipedia.org/wiki/RAID
but basically you would be looking at raid in 2 ways
raid 0 - striped
spreads your data over 2 or more hard drives. a real basic way to look at this is that you have two hard drives doing the work of one so twice as fast (well not twice as fast, but i said basic thinking here).
however if one hard drive gets corrupted then you lose ALL your data.
raid 1 - mirrored
this is 2 hard drives that both have the exact same data, but you only see one in my computer. you dont get any speed benefit, but you do have a backup when one of your hard drives fails
then thre is a lot of other ways of setting up raid, for example raid5 that combines the above, so you have performance and reliability. and others that can continue working if 2 or more hard drives fail at the same time.
i'd say the majority of home users dont really need raid. overclockers and benchmarkers would have it, some gamers use raid 0, and others use raid 1 if their data is critical, but you would have external backup anyway in that case. i guess if you were a big video editor then raid 0 would be handy.