Chemotherapy is the use of drugs to treat cancer throughout the body.
The effects of chemotherapy are systemic, because the drugs are transported via the blood stream to most parts of the body. Like normal cells, cancer cells grow and divide through the same life cycle. The difference is that cancer cells are uncontrolled and grow in an abnormal fashion. Chemotherapy drugs destroy cancer cells by stopping their growth so they can't divide and multiply at one or more points in their life cycle. Cancer cells that are actively dividing are the most sensitive to chemotherapy.