Friction always opposes motion between two surfaces, although it is not a fundamental force of the universe like gravity. The coefficient of friction is used to describe the ratio of friction between two bodies and the outside force pressing them together.
Several types of friction exist. Dry friction refers to the resistance of lateral motion between two dry surfaces. This resistance can either be static, when the bodies are not moving, or kinetic, when they are moving. Fluid friction, also known as viscosity, occurs when layers of a fluid are moving relative to each other. Lubricated friction occurs when a fluid separates two solid surfaces.