Pap smears are used primarily to detect cancer of the uterine
cervix or precancerous conditions - abnormalities of the cervix
that may lead to cancer. The key is early detection made possible
by microscopic examination of a specially treated sample of
cervical cells. Every woman should have an annual Pap smear as part
of a complete pelvic examination beginning when she becomes
sexually active or reaches 18 years of age and continuing for the
rest of her life.
Pap smears consist of cells removed from the cervix which are
specially prepared for microscopic examination. The cells are
removed by a gynecologist or other health care provider by brushing
or scraping the cervix during a pelvic examination. The removed
cells are evenly spread on one or more glass slides. Each slide
typically contains hundreds of thousand of cells. All Pap smears
should be sent to an accredited laboratory to be stained, examined
under a microscope, and interpreted.
(cut and pasted from dunno which website!) 
