Canker sores are also known as mouth ulcers, and they appear on the gums, roof of the mouth, cheeks and tongue. According to MedlinePlus, if you bite your. Nov 14, - A canker sore is a small ulcer inside the mouth. Canker sores usually appear on the soft parts of the mouth, such as the tongue, inside of the cheeks, and just inside the lips. The medical term for canker sores is recurrent aphthous stomatitis or RAS, and these ulcers will reappear again and again over time.