The purpose of any treatment method is to kill or remove cancer cells. Treatment is considered successful if the tumor is eradicated and you can live longer without cancer inside you. Unlike treatment, prevention strategies are aimed at a much earlier phase in cancer development. When prevention is successful, it stops cancer from developing in the first place. Obviously, prevention and treatment are intended for two entirely different groups of individuals.
Treatment is designed for people who've been diagnosed with a disease by their physicians.
Prevention, on the other hand, is meant for those who have yet to develop the disease and don't want to get it. The latter can occur in a variety of settings, depending on the desired result. Some preventive measures, such as polio vaccines, are administered to the general population. Others, such as cholesterol-lowering drugs, are given only to people with an increased risk for acquiring coronary artery disease. Scientists are now studying several agents that may one day be used to prevent prostate cancer, but they have yet to decide what will be chosen and who should receive it.