Who's asking: Steve Lechner, Freeport, ME
The grip is responsible for moving and setting up the physical equipment of filmmaking: lights and reflectors, Steadicam tracks, pipes and scaffolding, etc. The "key grip" is the leader of a group of grips (say that fast), and can also work as the on-set construction coordinator.
Don't make the mistake of thinking this is unskilled labor. Good grips can make all the difference to a day's shooting. If you've never been on a movie set, you would not believe how long it can take to set things up -- and if it takes too long, depending on the shot, the light has changed and you have to set it up all over again.
Since this was Steve's follow-up question, the "best boy" is the key grip's chief assistant, sometimes called the assistant chief lighting technician. It's the best boy's responsibility to figure out how many men and how much equipment is required for each day's shooting.
I'm off to Agway to look at seedlings. It's a gorgeous day, and Dizzy and I have already spent an hour down at the Cobbosseecontee, reading and chewing on sticks. (Well, each of us did one of those things.)