Mouth ulcers are open sores in the lining of the mouth and are also known as aphthous ulcers or canker sores. There are three main types of mouth ulcer: minor, major and herpetiform. The two main causes of mouth ulcers are from accidental damage (minor food burns for example) and stress. Viral infections such as herpes (cold sores) are also a type of ulcer. An ulcer has a white or yellow oval centre and an inflamed red border.