Cosmetically speaking:
The use of a scrubbing medium to remove dead skin cells, dirt, and other detritus from the surface of (uh... living) skin. Widely touted as an acne prevention measure, exfoliation is typically done daily or weekly (in the case of sensitive skins).
Technically, you exfoliate every time you use soap, since the use of any soap is mildly (I did say mildly) abrasive.
To go a step further, humans spontaneously exfoliate every second, simply by dropping dead skin cells in their wake. To observe a model of the phenomenon, hold a very pissed-off, soft-haired cat in the air for several seconds and observe the air around it.
The Cult of Exfoliation:
It is highly addictive. Freshly exfoliated skin feels wonderful. Keep an eye on your teenage daughters; that fresh, glowing look may be the door to a lifelong problem.
In many families, the introduction of an exfoliation regimen is a rite of passage. From babyhood baths, on to adolescent acne prevention products, exfoliation progressively encourages deeper dedication and time commitment through the years. Soap may be considered a "gateway exfoliator".
Clinique advertises exfoliation as the solution to the majority of problems afflicting any life form on earth. Since they also market some of the most powerful exfoliators available without a prescription, one wonders about their motives.
A recent television advertisement for St. Ives' Invigorating Apricot Scrub ("Gently exfoliates dull surface cells to reveal glowing, fresh, healthy skin") went so far as to admit the almost religious nature of the exfoliation experience. Don't ask me how I know what the Apricot Scrub bottle says. You have to know these things when you are investigating the exfoliation underworld.
As many beleaguered boyfriends will admit, strong exfoliators can be slightly painful, particularly when followed by astringents. I mean stuff like witch hazel and isopropyl alcohol. Go ahead, do it. I dare you. This adds to the whole rite of passage mystique.
You may find yourself offered free samples of exfoliant in shopping malls or other "safe" locations. Be aware of the risks. That is all.