Canker sores

An aphthous ulcer or canker sore is a type of mouth ulcer which presents as a painful open sore inside the mouth, caused by a break in the mucous membrane. The condition is also known as aphthous stomatitis, and alternatively as "Sutton's Disease," especially in the case of multiple or recurring ulcers.
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Canker sores are small sores or ulcers that appear inside the mouth. They are painful, self-healing, and can recur. [more]Canker sores - Community and Resources

Also known as aphthous ulcers, these are small ulcer craters in the lining of the mouth that are frequently painful and sensitive. Canker sores are one of the most common problems that occur in the mouth. About 20% of the population (1 out of 5) people will have canker sores at any one time. Canker sores typically last for 10-14 days and heal without scarring The word canker comes from the Latin cancer for crab. (The Latin cancer was once pronounced kanker from which came canker). Chronic ulcers might seem as hard as a crab shell.