The branch of medicine concerned with the study, diagnosis, treatment, and prevention of cancer is Oncology.

Cancer is not a single disease. It is a group of diseases, all of which display the common feature of cells showing uncontrolled growth. Cancer may affect people at all ages, children and adults but the risk increases with age.

The cells of all animal bodies follow a predictable pattern, wherein old cells undergoing wear-and-tear, in the course of their function, become defunct, and eventually die, with new cells taking their place.

The essential difference is the time it takes for cells to wear out and die, and this varies between cell types. Some have a natural lifespan of a few days like red blood cells, or epidermal cells (i.e., cells on the skin surface), while others like brain cells, may survive for a lifetime. This is regulated by a genetic preset mechanism, whose codes are present in every cell in the body. Almost every cell dies and is replaced, except certain specialized cells like brain cells. When this process goes out of the normal synchrony, cells multiply out of control with a resultant malignant process. The end result is a mass of cells, called a tumor.

Some tumors are 'benign' or non-cancerous. A benign tumor does not spread, or metastasize to other parts of the body. They do not invade or destroy adjacent cells or tissues, nor migrate to other locations via lymph or blood. Malignant tumors damage adjacent tissues and invade other tissues and organs. When malignant cells break away from the primary tumor and settle into another part of the body, like the liver, lungs, bones, or the brain, the resulting new tumor site is called either a metastasis or a secondary tumor.

There are several major pathological types of cancers: carcinomas form in the cells that cover the skin or line the mouth, throat, lungs and organs; sarcomas are found in the bones, muscles, fibrous tissues and some organs; leukemia cell types are found in the blood, the bone marrow, and the spleen; and lymphomas in the lymphatic system.