Before answering the question of what is causing canker sores, we need to define the condition.
The inside of the mouth is covered by a membrane composed of a layer of epithelial cells called the Epithelium tissue. When an area of this membrane is damaged it forms an ulcer (basically a hole) leaving the specific area unprotected and exposed.
Food and drinks, teeth or the tongue that come in contact with the formed ulcer result in a painful experience, sometimes mild but most of the time acute pain. Canker sores are ulcers, however all ulcers are not canker sores. There are cold sores caused by herpies virus and canker sores (aphthous ulcers), which are not contagious and seem to appear out of nowhere.
Canker sores usually occur on soft tissues, at the inside of the lips, cheeks and sometimes on the tongue. Rarely they appear on the gums or the roof of the mouth.
They last 5 to up to 10 days and appear as red irritated spots of skin, most with a white spot in the center and/or white border. They vary in size between 1-4mm. Larger ones are possible but rarely seen. Large ones have irregular shape as well.
Almost all people will experience a canker sore in their lifespan, most will experience one once per year or so. Rarely one will experience more than one in sequence over a short timespan. In that case it may be the result of an allergy or a symptom of a disease and should receive the advice of an expert.
Statistically speaking, canker sores are most commonly found on women and in the age group of 8 to 40. It is important to note that they may appear at any age and most of the time the they are do not require special care.