Friction

Friction acts between touching surfaces and opposes their relative movement. When a sledge slides over the ground, friction from the ground opposes the sliding. Friction is not always unhelpful: friction between your shoes and the ground helps you walk without slipping.

Brakes produce friction that slows the wheels, while friction between the tyres and the road provides the grip needed to slow the vehicle. Braking transfers the vehicle's kinetic energy, the energy it has because it is moving, mainly into thermal energy.