He (or sometimes she) works for the director of photography, setting up all the lights as they are needed for shooting, directing a crew of other electricians, preparing the basic lighting for the next scenes to be shot, ordering all lighting equipment and supplies, and so forth. And guess who his or her top assistant is? Yes, the "best boy." Sexist but accurate, since they're almost always men.
"Grips" are the crew members who carry and set up equipment around the set or location. On most productions, the head grip will be called the "key grip," and on very big productions, where two crews will be shooting at the same time, there will be more than one key grip. The camera crew has its own grip, the "dolly grip," the person who sets the camera up and then pushes the camera on its dolly or tracks; there are grips who specialize in operating the camera crane when that is called for. Other grips will work with the carpenters to get and place the materials needed to assemble the sets. When the gaffer calls for particular lighting equipment, it's the grips who bring it and set it in place for the electricians to light and focus. Sometimes key grips are called "best boys," too, which can make for confusion. A "wrangler" handles the animals, as you might expect, but there's no size limit. The film business has room for "mouse," " marmot," and "cockroach" wranglers, as well. (If you've seen a credit for a "baby wrangler" it's the in-joke way of crediting the registered nurse who, by law, has to be with an infant performer on the set.) The "greensman" or woman is responsible for preparing and maintaining the live or fake flowers, foliage, trees, cornfields, or wastelands you might see in the film.
Don't mistake odd titles for lack of skill. These are lifelong professionals who do extremely difficult jobs quickly and with very few mistakes; millions of dollars -- and sometimes the safety of actors and other crew members -- ride on how well they work.
