Layer Masks and Splices...

I've noticed that some people have suggested using layer masks to get a certain effect. And I tried to figure out what a layer mask is exactly. Obviously I've never used one, so I was kind of curious to learn what one is. Which brings me to the question:

What is a layer mask, how would I do it, and what effect would it create or in what instance would you use one?

I just figured it would be good to know. Oh! And I came across the splice tool in my PS and I don't know what that's used for either. Any ideas?

Thanks so much!