American Football: Snap Count Questions?

How exactly does the snap count work? I know it's different for each team, and I know you might say "on one or one two" which means first or second hut, but is there more to it? What about the calls like blue 18, etc... are they audibles? Does the team know when to snap after a certain amount of "blue"'s, or whatever? Because it seems like some teams, patriots for example, tom brady almost always goes on the first "set", and you'd think defense would key into that, but is there another way the timing works? Any details from someone who knows or plays football would be appreciated.