The best boy can be either in the electric crew or the grip crew. Depending on the case, he can be the best  boy electric or the best boy grip...He has many things to do. Mostly, he is responsible that everything is working just fine and the rest of the crew from his department can do their job well.

The "best boy" is very active on the movie set. He has to run from one side of the set to the other and make sure everything is functioning.