Friction is a tangential force that acts parallel to surfaces in contact and moving relative to each other. There are two types of friction: static and kinetic. Static friction resists motion of stationary objects, while kinetic friction resists motion of objects already moving. Friction depends on the normal force perpendicular to the surfaces and the coefficient of friction, which varies based on the nature of the surfaces. Laws of static friction state that friction is proportional to the normal force, independent of contact area, and the coefficient of static friction is greater than kinetic friction.