In brief, screening is a process that looks at a group of people who have no symptoms, who otherwise feel fine, to see if the early stages of a disease, like a cancer, is present.
Cervical cancer is a disease that develops only after a number of years in the pre-cancer stage.
It makes sense to try and find cervical pre-cancer rather than waiting until cervical cancer has developed. Finding cervical cancer before it has spread to other parts of the body has saved tens of thousands of lives over the past 40 years. With the introduction of the Pap smear in the late 1940's, women with cancer were found earlier, with their cancers localized only to the uterus or to the tissue immediately adjacent to the uterus. The number of deaths from cervical cancer decreased dramatically as a result. After this decrease, however, there has been a leveling off in the incidence of cervical cancer and the number of women dying from it.