Cells are the building blocks of living things. Cancer grows out of normal cells in the body. Normal cells multiply when the body needs them, and die when the body doesn't need them. Cancer appears to occur when the growth of cells in the body is out of control and cells divide too quickly. It can also occur when cells “forget” how to die.
There are many different kinds of cancers. Cancer can develop in almost any organ or tissue, such as the lung, colon, breast, skin, bones, or nerve tissue.
There are many causes of cancers, including:
- Benzene and other chemicals
- Certain poisonous mushrooms and a type of poison that can grow on peanut plants (aflatoxins)
- Certain viruses
- Radiation
- Sunlight
- Tobacco
However, the cause of many cancers remains unknown.
http://health.nytimes.com/health/guides/disease/cancer/overview.html