Just About It gave a good overview. I would like to add:
The radiation is used to bombard the area where the cancer is determined to reside (Either with an external device, or inserting radioactive materials into the afflicted area). This treatment is very painful and difficult on the body as it requires the destruction of not just the cancer but healthy cells in the area as well.
The effect of using radiation is damage on the genetic level. The DNA of the cancer and typically the cells in proximity is damaged or destroyed. It does not, nor is it expected to, typically work. "Cancer survivors" are the people who have lived for longer than the expected life span of a cancer victim. (There is much debate over this practice.)
For a comprehensive look at the official medical industry data check the National Cancer Institute's official site here: http://www.cancer.gov/cancertopics/factsheet/Thera...
If you want alternative treatments to cancer, search online and be persistent. Doctors often cannot help as they are bound by medical guidelines to follow "proper procedure" that is dictated by our greedy medical industry.