graphics Terms?

I'm curious about the terms that are used with graphics. What do the following mean in relation to each other:
royalty-free
stock
public domain
etc.

Which can be used in commercial use, which can't?

Uh, I don't think any of those have to do with graphics...

Unless you're talking about the legal info about the graphics such as copywriting and stuff...

Yah! That is what I was wondering about. Which can be used commercially, which can't?