Answer:

The force of friction occurs when one not IDEALLY smooth object slides against another one. This force always directed against the movement and, in absence of other forces, slows down the sliding.

The force of friction depends, MAINLY, on two factors: material the OBJECTS are made of (more or less smooth) and how tightly they are pushed against each other. Typical textbook problem in physics is to determine the force of friction when one object (say, a wooden block) slides horizontally across some SURFACE (like a table) and it's pushed against this surface by its weight (force of gravity). A variation is a SIMILAR problem, but the surface is not horizontal, but is a slope of some angle.

While the measure of how tightly the objects are pushed against each other is easily measured using the pressure force, the factor of material objects are made of is not so easily accounted for. For this physicists are using the coefficient of friction, which measures the force of friction between two objects made of certain materials per unit of pressure. It's determined experimentally.