It is the nature of cells to divide and increase their number in a process called mitosis. Normal cells divide to replace those lost, or to repair injuries only, then stop dividing.
Cancer is an abnormal, continuous multiplying of cells.
The cells divide uncontrollably and may grow into adjacent tissue or spread to distant parts of the body. Adenocarcinomas are cancers that form on a glandular surface, such as the lung, breast, prostate, ovary, or kidney. Sarcomas are cancers that occur in supporting structures, such as bone, muscle, cartilage, fat, or fibrous tissue. Leukemias and lymphomas are cancers that occur in blood cell elements. Brain cancers, nerve cancers, melanomas, and certain testicular and ovarian cancers do not fall into a general category.