An ulcer is a kind of open wound. Ulcers can be on surfaces of the body. They can be on skin. They can be mucous membranes like the surface of the stomach or inside the mouth.
Ulcers start when the top layer (surface) of skin or mucous membrane is damaged. The top layer dies. When it dies, the skin or mucous membrane opens. This leaves an open sore called an ulcer.
Ulcers can be from infection with germs. Ulcers can be from pressure on a part of the skin. Ulcers can be from substances that will burn skin or mucous membranes. Peptic ulcers are sometimes from burning by the acid in the stomach. Ulcers can be from cancer. Ulcers can be from diseases of blood vessels. If not enough blood goes to a body part, it can get an ulcer. Stress can also sometimes cause ulcers. Ulcers are often very painful.