Friction happens when one object slides along another object. It is the force that resists the relative motion between the two surfaces. Friction occurs in the opposite direction of the moving object's motion and reduces its speed.
Friction happens because very few objects have perfectly smooth surfaces. The rough edges on each surface bump into each other, causing friction. Different types of friction exist. When one object slides along an unmoving object, it creates sliding friction. An object moving through air or fluid results in fluid friction. A rolling object, like a wheel, creates rolling friction, which is less powerful than sliding friction.