Gang Boss: I'm pretty sure sometimes they are just called construction foremen, but why would you call yourself that if you could go by GANG BOSS. Their job is to oversee the construction crew and make sure everything on set gets built as it should.
Gaffer: The chief electrician and, hopefully, the DP's bestie.
Grip: They build and maintain everything that the camera goes on like a dolly or a crane, and also help set up all the stuff that the lighting equipment goes on.
Producer: What they actually do is totally up for the grabs -- either they handle the money, or they're the boss, or they're just also someone with the aforementioned golden ticket.