Mouth (Oral) Ulcers:

Mouth ulcers, also called canker sores, aphthous stomata and aphthous stomatitis, mostly occur on the inner cheek, inner lip, tongue, soft palate, floor of the mouth, and sometimes the throat. They are usually about 3-5mm in diameter - though sometimes significantly larger, and often appear 2 or 3 at a time - though sometimes there can be 10-20 or more.They often start by themselves as a small bubble or blister (this stage is easy to not notice) which then becomes open and ulcerated, which is when the pain really starts. Alternatively they are caused by trauma to the lining of the mouth (e.g. by accidentally biting one-self).  A reddish halo usually surrounds the lighter colored centre, which upon closer examination is seen to be a pit or hole.The worst thing about mouth ulcers is the pain that they cause, which is constant and made worse by eating, drinking and talking. The picture of the homunculus that you may recall from schooldays shows how very sensitive the mouth and tongue are compared to most other parts of the body - which goes some way to explain the amount of discomfort caused by something so small.

Severe ulcers can cause the sufferer to avoid eating, drinking and talking (and kissing), which can be awkward socially. Speech can be very painful resulting in a loss of clarity or enunciation - and sometimes spitting, which doesn't normally go down too well. Some sufferers get worn down by the pain and become fatigued and depressed, and so I have been told, irritable too. No specific single cause has yet been isolated, though it seems they are not caused by infectious agents such as viruses or bacteria and are therefore not contagious.

A current popular theory is that they are linked to the auto-immune system, in that certain triggers (that may be different from one person to the next, and could also be deemed an allergic reaction) cause the mucosal lining (protective layer on the cheeks, gums, tongue, throat etc.) to become compromised, such that it is attacked by one's own saliva.

This does go some way to explain why a definitive "cause" and "cure" is proving so elusive, but does not provide comfort for chronic sufferers of the painful things.

If you have a mouth ulcer that is causing you worry, visit your doctor. 1 in 5 people get mouth ulcers, so the chances of it being anything significant are very remote. The tests for allergies and the big C are quick and easy - so if you are not sure, get it checked.

Sources:
http://www.mouthulcers.org/intro.html  http://www.medicalwellnesscenter.com/AphthasolCankerSoreWelcomepage.html