Friction is the force between two surfaces which opposes their relative sliding motion. The microscopic physics which produce this force are not well-understood, and its behaviour is highly complex, both theoretically and experimentally. The study of friction is intimately connected to the study of lubrication and of wear; the discipline is known as tribology.
Friction-like resistance can occur in other contexts; fluids experience viscosity, solid objects moving through fluids experience fluid friction, and sound waves in solids, liquids, and gases lose energy as they propagate.