When a body is in motion, it often experiences resistance because the body interacts with its surroundings. Consider the case of sliding down a sandy hill compared to an icy mountain. The resistance you experience from the sand is a force of friction. You can read more about the microscopic phenomena of forces responsible for this difference.

Friction opposes relative motion between systems in contact. Friction is a common yet complex force. The direction of the friction is always opposite that of motion, parallel to the surface between objects, and perpendicular to the normal force. An object can experience both static and kinetic friction.