The word chemotherapy sometimes stirs up strong emotions in newly diagnosed cancer patients; but in some cases, chemotherapy is the best method of treating widespread disease. And in other cases, chemotherapy has been a life saver. Chemotherapy drugs are used to treat systemic cancer for those whose cancers may have metastasized or are at high risk of metastasizing; and for cancers that effect blood or lymphatic systems. It is used for curative purposes, as a preventive for metastasize, and to slow tumor growth. It is a treatment regimen that sometimes produces toxic effects, but there are many adjunct medications in use today to help with these conditions.
This is an area of cancer treatment that is ever changing. Chemotherapeutic drugs are used independently, in combinations with other chemotherapy drugs, and in combination with other types of drugs; creating a complex variety of regimens. How they target the cancer cells for destruction is multi-faceted and rather complicated to the layman. I wouldn't begin to try to explain it with my limited knowledge. But, if you would like to read further, here are some good starting points:
American Cancer Society - part of their Making Treatment Decision series.
CancerHelp UK - general information about chemotherapy.
National Library of Medicine - explains all aspects of this treatment.
Oncology Channel - overview of chemotherapy and list of side effects, by drug.
One relatively new aspect in the area of chemotherapy drugs are some advances in delivery of the drugs. One such advance is chemoembolization. Chemotherapy drugs are injected directly into the center of the tumor site. Then the artery flow is blocked by infusing particles into the blood vessels supplying the tumor. This, in essence, locks the chemotherapy drugs inside the tumor. This type of procedure is used on liver cancers. Another advance, designed at MD Anderson is an aerosol delivery system; which shows promise.