One type of precancer

Actinic Keratosis
Actinic keratosis (AK), also known as solar keratosis, is the result of prolonged exposure to sunlight. It is a small crusty, scaly or crumbly bump or horn that arises on the skin surface. The base may be light or dark, tan, pink, red, or a combination of these... or the same color as your skin. The scale or crust is horny, dry, and rough, and is often recognized by touch rather than sight. Occasionally it itches or produces a pricking or tender sensation. It can also become inflamed and surrounded by redness. In rare instances, actinic keratoses can bleed.
The skin abnormality or lesion develops slowly and usually reaches a size from an eighth to a quarter of an inch (2mm to 4mm) but can sometimes be as large as one inch. Early on, it may disappear only to reappear later. It is not unusual to see several AKs at a time. AKs most likely appear on the face, lips, ears, scalp, neck, backs of the hands and forearms, shoulders and back — the parts of the body most often exposed to sunshine. The growths may be flat and pink or raised and rough.