Use the link in JFraz' signature above on layer masks. I didn't get them when I first started either but once I finally got through that tutorial I began to use them all the time. I really can't thank JF enough for that one. Now that I think about it I need to go make sure I rated that tutorial.
On a side note, I kind of suck at following tutorials because I mostly just like figuring things out for myself but every now and then you just gotta do it.