Canker sores are small, painful lesions that can form inside the mouth. They are also known as aphthous ulcers and are usually white or yellow in color with a red border. LESION is a medical term for any abnormal tissue change, such as an open sore or wound. An ULCER is an open sore or LESION that is caused by a break in the skin or mucous membrane. Canker sores are a type of ULCER that can occur in the mouth, often on the inside of the cheeks or lips.