Boob is a word for a woman's breast — it's also a word for an idiot. So if you're staring at a lady's boobs, you probably look like a boob.
This is a very common slang word that's not really inappropriate. Nobody is going to get too mad about you using the word boob, though breast is definitely more anatomically correct. The other meaning of boob is similar to dope, dumbbell, dummy, pinhead, and numbskull. This meaning is also pretty harmless. Calling someone a boob is only a little insulting — not quite fighting words.