Friction

Friction is the use of a rough-surfice to buff away hair from the surface of your skin.
You can use anything from a pumice stone to a specifically designed hair removal mit.
Different products are available for different areas of the body, though the most common areas are the legs and arms as the skin is flat and fairly durable. However, specific firction products are available for areas such as the face and bikini area as well.
Lots of people like to use friction as an upkeep method between waxing sessions.

FRICTION - HOW TO DO IT
When doing the legs or arms it's a good idea to take a warm shower and exfoliate the area, otherwise you may have problems with dead skin sloughing off as you treat the area. A shower also helps to soften the hair, making your job easier.
Next, take your pumice or mit and rub it gently on the area using slow, circular movements. It is important to be very gentle. Remember you're not sanding splinters off a wooden board, you're treating your skin!
Your strokes should have only the slightest amount of pressure. It may feel like you're not doing anything, but if you check you will see the hair being buffed away.
Afterwards you may want to apply soothing lotion in case your skin becomes irritated, but it's a good idea to stay away from irritants such as heavily perfumed lotion for a day or so.

FRICTION VERSUS OTHER METHODS
Friction is fairly cheap, with the friction mits going for about $5.50, and $5.00 for pumice stones.
However it is short-term, if you use friction you'll probably see some hair regrowth by the next day. But it does not cause stubble like shaving, as the hair is rubbed away rather than sliced, so no sharp edge is formed on the hair.
It is mostly painless if done correctly, unless you have sensitive skin in which case you may not be able to rub the hair away without irritation. If this is the case you may want to find another method.