The term "Grip" dates back to the early days of theater and there are a few theories of how Grips got their name.
Some have suggested the name comes from the 1930s–40s slang term for the technicians "grip" on a black tool bag they would carry to work. Listen to the first line of this Bing Crosby song... https://www.youtube.com/
Others claim it came from stagehands who used grips or handholds that were attached to pieces of scenery and another more likely theory states that in the days of hand-cranked cameras, it would be necessary for a few burly men to help setup the cameras and hang on to the tripod legs to stop excessive movement of the camera. These men became known as the "good grips"—as they were constantly being instructed to "keep a good grip on the tripod". They later evolved to supporting the camera in many different ways.
Grips are a jack of all trades...In the U.S.,Canada and other parts of the world they do lighting and camera support with some construction and advanced rigging skills. In the UK, Australia and most parts of Europe Grips are solely responsible for camera movement and camera support with some rigging assignments.
There are many types of positions in the Grip department such as a Key Grip, Best Boy Grip, Company Grip, Dolly Grip, Rigging Grip, Construction Grip and even Marine Grips.
Every position holds an important role on set and although the work is physically demanding and the hours are usually long, the work can be very rewarding.