How do laws work internationally?

If something is illegal in the US, but legal where I live, and you'd do that thing (to make this example easier, let's say it's online as that's the easiest way to commit crimes internationally I guess) and then with something that's protected by American law, would you be commiting a crime and be able to get in trouble? I've always wondered how that works, and if an extradition treaty plays a role or something