[IronJack]

Carries props, sets, and other equipment. A KEY GRIP is the boss man. Now find out what a "best boy" does.

[Tennessee_Bob]

Now find out what a "best boy" does.

Lots better than the worst boy??

[Drew68]

From the Internet Movie Database Movie Terminology Glossary

Best Boy: The chief assistant, usually of the gaffer, but more often lately used as a general term for the second in command of a group.

BTW, this is a great site. It will tell you anything you ever need to know about films.

Also from the IMDb,

Grip: In the USA, a grip is a person responsible for the adjustment and maintenance of production equipment on the set. Their typical duties include laying dolly tracks or erecting scaffolding. In the UK, grips work exclusively with equipment that the camera is mounted on.