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, continual multiplying of cells.
The cells divide uncontrollably and may grow into adjacent tissue or spread to distant parts of the body. The mass of cancer cells will eventually become large enough to produce lumps, masses, or tumors that can be detected. Adenocarcinomas are cancers that form on a glandular surface, such as the lung, breast, prostate, ovary, or kidney. Leukemias and lymphomas are the 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.