Cancer is the catchall term applied to diseases caused by the uncontrolled growth of abnormal cells. Cancer isn't one disease. It is many different diseases, more than 100 and counting.
Each kind of cancer is usually named for the cell type in which it begins — cancer that starts in a lung is called lung cancer; cancer that starts in pigment cells in the skin, which are known as melanocytes, is called melanoma.
When detected and treated early, cancer can often be stopped. That said, cancer is a leading cause of death and disability around the world.