A grip is independent department on set that builds and operate complex equipment such as dolly, tracks, sliders, cranes and everything that supports the cameras.
Key Grip is the head of Grip department. He/She goes on technical scouts and makes solutions for camera positioning.
Best Boy is a grip guy who is a right hand to Key Grip. Takes care of communication within team and with production.
Dolly Grip is a specific person that operates the camera dolly.
Dolly is a wheeled cart that camera is usually mounted on. It has hydraulic arm, that can move vertically. It can be used on tracks to move horizontally as well.
Grip Truck is a custom truck for grip equipment. Usually needs to take a lot of load.
Apple box is a box made out of wood and serves for underlying the camera or other grip equipment. In bigger elevation is used to underlay tracks.