What's the difference between a "mole" & an "rat"?

I'm speaking figuratively, not about basic zoology. Would both b synonyms for "snitch" or "2x agent"? Also, does the mole or rat have 2 reveal 2 one party that he or she belongs 2 the other party? Could the other party discover an mole or a rat? Incidentally, I was watching "The Departed"; liked the final scene w/ the mouse.