Cancer is the result of cells escaping normal control mechanisms, and growing uncontrollably as a result. It is a result of genetic changes--certain genes normally regulate the cell cycle and mutations that alter these genes can cause cancer. Cancer is often caused by mutations resulting from chemical carcinogens, certain viruses, or radiation. These mutations can cause cancer if they occur in proto-oncogenes.
A proto-oncogene is responsible for stimulating cell growth and division, and so a genetic change may result in one becoming much more active, transforming it into an oncogene, which will promote excessive cell growth--cancer. Another important gene is the tumor-supressor gene; it inhibits abnormal cell division. A mutation may cause one of these genes to malfunction, also resulting in effects similar to those of the oncogene.
Another common mutation activates the creation of telomerase. This enzyme prevents the degradation of the ends of chromosomes at each cell division, removing the natural limit on the number of times a cell may divide (programmed cell death).
One such mutation is usually not enough to cause cancer. There must be several, usually in proto-oncogenes, tumor-suppressor genes, and the gene for telomerase, before a tumor cell will arise. However, these mutations accumulate over the lifespan of the individual (and so older individuals are more likely to have cancer), and some can even be inherited (this is the genetic basis for increased likelihood of cancer).