What does "Key Grip" entail? You're a lighting guy, right?
I think the key grip is the boss of the grip department. Grips do a lot of work with the camera crew, sort of working with the positioning of cameras I think. Grips do some work with the lighting, but most of that stuff is handled by the gaffer.
Yeup that's a pretty good description. I'll go into more detail tomorrow, doing a poptart commercial tonight.
Alright! Here's a logner-than-most-will-find-interesting description of what a grip does on a movie set.
Spoiler because this part got long and boring.
At the top of the lighting department is the director of photography, the DP. He is in charge of the look of the film, which means he's in charge of the camera department as well as lighting. Next is the Gaffer, who takes what the DP wants lighting wise and figures out how to logistically implement that by deciding what lights to use, how to run power to those lights, and so on. He then the gaffer delegates his or her plans to the electrics who move a bunch of heavy cable and lights. Which is my job, I'm usually an electric.
Now technically the gaffer is the key grip's boss, but don't tell that to the key grip. Here is where we get into a lot of weird union rules about job description that don't make any sense outside of the industry, but it does help get things done faster if you follow them, and movies are all about getting things done faster.
So, the difference between an electric and a grip: electrics run power to and operate the lights (they also run power to anything else that needs power). Grips setup the rigging to get the light where it needs to be, be it mounted to the ceiling, on a huge JLG lift, or inside a wall. Grips are also in charge of subtracting light using flags and nets. In other words, they put pieces of black in front of lights to make shadows and keep the light from going where the DP doesn't want it to go. Those pieces of black (or white or net or whatever) can be small, or friggin huge if you're shooting outside. They handle the part of lighting that doesn't require electricity, so they will also bounce light with pieces of white. (And they pay off pirates with pieces of eight).
As Thrifty pointed out, grips also work with the camera, though I would say both parts of their job are equal. Just as they are in charge of mounting the lights where they need to go, they are in charge of putting the camera where it needs to go if it's somewhere that a tripod won't do. So they are in charge of dollies and cranes and jibs and doing car rigs and all that jazz.
And the key grip is in charge of the grips. As I said, I'm usually an electric, but on smaller budget movies the line between electrics and grips isn't as important because they don't have the huge armies of crew members to delegate tasks to that union movies have to deal with.