Friction is an everyday force that arises from two surfaces interacting. When these surfaces slide against each other, this interaction increases the thermal energy of the two surfaces (the temperature goes up). While it is easy to think of friction as a 'bad' thing, friction is needed in order to drive (it's what pushes the wheels on our car forward and allows us to stop and turn), or even walk.
Friction is why:
Friction in engines and machines contributes to losses of useful energy, which also wears out the parts in a car (hence the need for lubricating oil). This means that friction is taking the large (macroscopic) motion of an object and turning it into the small (microscopic) motion of atoms and molecules. Friction is a non-conservative force meaning the energy becomes less useful (but doesn't disappear from the universe, see conservation of energy).
There are three types of friction:
For a more complete description of physics please consult hyperphysics.