Recurrent Ulcers of the mouth mucosa are termed Aphthous Ulcers. The oral mucosa is the non keratinized part of the mouth. They are ulces that often occur either solitarily or in a small group, under the tongue, on the inside of the cheek or below the teeth on the mucosa. They never appear on the gingival and if smaller multiple ulcers appear on the gingival they are usually herpetic lesions. Aphthous Ulcers can be large and are commonly more than 5mm in diameter
Their cause is unknown , but stress and/or a weakening of the immune system is thought to play a role in their formation.
Although aphthous ulcers, are annoying and can interfere with our enjoyment of eating, they usually are not serious and tend togo away on their own without any intervention in about two weeks. No treatment is necessary. If a sore in the mouth lasts more than two weeks, you should see a dentist or a doctor!. Also if you have swelling or fever associated with an ulcer, it is a good idea to not wait two weeks and to visit your dentist sooner.
Obviously, not all oral ulcers are Aphthous Ulcers and some other causes of ulcers are trauma, bacterial and viral infections. Also mouth ulcers can be caused by burns(hot pizza or very hot tea or coffee). These burns usually occur on the palate and not as commonly on oral mucosa.
Here are some good links about Aphthous Ulcers
http://en.wikipedia.org/wiki/Aphthous_ulcer
http://www.ada.org/public/topics/mouth_sores.asp
