A cold sore is the outward symptom of an outbreak of (and previous infection by) the herpes simplex virus, probably the type 1 strain. Cold sores are the most common form of herpes simplex infection and are otherwise known as "herpes labialis", which is the infection that occurs when the virus comes into contact with oral mucosa tissue or abraded (scratched) skin.
A canker sore, otherwise known as an aphthous ulcer, is a type of painful oral ulcer that typically occurs inside the mouth (normally on the inside of the lip) or in the upper throat. The disease is also known as "aphthous stomatitis", and when you keep getting it over and over again (it keeps recurring), that particular condition is known as RAS: Recurrent Aphthous Stomatitis, and it's one of the most common oral conditions since around 10% of the population is afflicted with it.
The fastest way to distinguish a canker sor from a cold sore, 98% of the time straight away, is this: cold sores almost always occur outside the mouth, it's extremely rare that that they are ever found inside, whereas canker sores always occur inside the mouth.
Canker sores are generally classified into three groups based on size:
1. Minor sores have a diameter of 1-10mm. They are the most common (80% of all canker sores) and usually only last about 7 to 10 days.
2. Major sores (10% of canker sores) will have a diameter of more than 10mm and they may take anywhere from 10 to 30 days to heal. Also, they may leave a scar after they heal.
3. Herpetiform ulcers (the other 10% of all canker sores) are formed by a cluster of multiple small individual sores (less than 3mm each). They also normally heal within 7-10 days.
The most effective current treatment of canker sores has been found to be Vitamin B-12, which is just as effective regardless of whether or not there is a B-12 deficiency present in the patient. Also, anti-biotics and anti-virals are sometimes effective, depending on the situation, but these are rarely prescribed. One of the best things to do to avoid canker sores is to avoid spicy foods and to practice proper dental hygiene on a regular basis.