kinetic friction
Kinetic friction is the drag an object feels while it is actually sliding across a surface. Push a book across a table and it slows and stops; that resisting force, opposing the sliding, is kinetic friction. It always points opposite to the direction of motion.
Kinetic friction acts on an object that is sliding, parallel to the surface and opposite to the velocity. Its magnitude is f_k = mu_k times N, where mu_k is the coefficient of kinetic friction and N is the normal force pressing the surfaces together. Unlike static friction, it is roughly constant while sliding, and it does not depend much on how fast the object moves or on the contact area.
Kinetic friction converts kinetic energy into heat, which is why sliding things slow down and why rubbing your hands warms them. It is usually a little weaker than the maximum static friction (mu_k < mu_s), so once an object starts moving, a smaller force can keep it going at steady speed.
A 10 kg crate slides across a floor with mu_k = 0.3. Kinetic friction on it is f_k = 0.3 times (10 times 9.8) = 29.4 N, pointing backward against the motion, steadily slowing the crate.
While sliding, friction has a roughly constant value, mu_k times N, opposing the motion.
Kinetic friction dissipates mechanical energy as heat, so it is a nonconservative force. It is very nearly independent of sliding speed and of the apparent contact area, a surprising but well-tested result.