Canker sores, also called aphthous ulcers, are small, shallow lesions that develop on the soft tissues in your mouth - under your tongue, inside your cheeks or lips, and at the base of your gums. Unlike cold sores, canker sores don't occur on the surface of your lips and aren't contagious. They can be very painful, however, and can make eating and talking difficult.
Most canker sores go away on their own in a week or two. Check with your doctor or dentist if you have unusually large or painful canker sores or canker sores that don't seem to heal.
From www.mayoclinic.com