An ulcer is a SORE or LESION on the skin or mucous membrane, usually caused by an infection or inflammation. FESTER, SORE, CANKER, ABSCESS, and chancre are all terms used to describe different types of ulcers. Festering is the process of a wound becoming infected and producing pus. A SORE is a painful area of skin that may be swollen or inflamed. CANKER is an ulcer that is typically found in the mouth. An ABSCESS is a collection of pus that forms in the tissue due to bacterial infection. A LESION is an area of abnormal tissue that results from injury or disease. Finally, a chancre is a painless ulcer that is usually the first sign of a sexually transmitted infection.