A cancer is an uncontrolled proliferation of cells. In some the rate is fast; in others, slow; but in all cancers the cells never stop dividing. This distinguishes cancers — malignant tumors — from benign growths like moles where their cells eventually stop dividing (usually). Even more important, benign growths differ from malignant ones in not producing metastases; that is, they do not seed new growths elsewhere in the body.
Thumbnail: This is a photograph of a basal cell carcinoma on the back taken by me. Basal cell carcinoma is the most common skin cancer. Image used with permission (Public Domain; ).