Friction is the opposing force that prevents relative motion between two surfaces in contact. There are two main types of friction: static friction and kinetic friction. Static friction acts when an object is at rest or trying to move, while kinetic friction acts when the object is already in motion. Friction is described using coefficients of friction which relate the frictional force to the normal force between the surfaces. The coefficient of static friction is typically larger than the coefficient of kinetic friction. Frictional forces follow laws including that friction always opposes motion and is proportional to the normal force.