That's why the friend you're talking to on a New York street is not drowned out by the traffic and crowd noises. On film, the sound effects editors and re-recording mixers have to do the focusing. First they record the dialogue and later add other sounds at just the right levels. The boom operator moves the boom as necessary. Now it refers, to the head of the electrical department.
Gaffers work closely with grips, who are the folks responsible for the production equipment.
The key grip is, of course, the person who holds the keys. Actually, the key grip is the chief grip. The best boy is the chief assistant to the gaffer, or the second in command of another group.