Friction is the resistance to motion of one object moving relative to another. It is not a fundamental force, like gravity or electromagnetism. Instead, scientists believe it is the result of the electromagnetic attraction between charged particles in two touching surfaces.
Scientists began piecing together the laws governing friction in the 1400s, but because the interactions are so complex, characterizing the force of friction in different situations typically requires experiments and can't be derived from equations or laws alone.
For every general rule about friction, there are just as many exceptions. For instance, while two rough surfaces (such as sandpaper) rubbing against each other sometimes have more friction, very smoothly polished materials (such as plates of glass) that have been carefully cleaned of all surface particles may actually stick to each other very strongly.
Types of friction
There are two main types of friction, static friction and kinetic friction. Static friction operates between two surfaces that aren't moving relative to each other, while kinetic friction acts between objects in motion.
In liquids, friction is the resistance between moving layers of a fluid, which is also known as viscosity. In general, more viscous fluids are thicker, so honey has more fluid friction than water.
The atoms inside a solid material can experience friction as well. For instance, if a solid block of metal gets compressed, all the atoms inside the material move, creating internal friction.
In nature, there are no completely frictionless environments: even in deep space, tiny particles of matter may interact, causing friction.
Coefficient of friction
Two solid objects moving against each other experience kinetic friction. In this case, the friction is some fraction of the perpendicular force acting between two objects (the fraction is determined by a number called the coefficient of friction, which is determined through experiments). In general, the force is independent of the contact area and doesn't depend on how fast the two objects are moving.
Friction also acts in stationary objects. Static friction prevents objects from moving and is generally higher than the frictional force experienced by the same two objects when they are moving relative to each other. Static friction is what keeps a box on an incline from sliding to the bottom.