From: ada.org

What's the difference between a canker sore and a cold sore?

Canker sores are often confused with cold sores. An easy way to distinguish between the two is to remember that canker sores occur inside the mouth, and cold sores usually occur outside the mouth.

A canker sore (also called aphthous ulcers) is a small ulcer with a white or gray base and red border. There can be one or a number of sores in the mouth. Canker sores are very common and often recur.

A cold sore, which is also called fever blister or herpes simplex, is composed of groups of painful, fluid-filled blisters that often erupt around the lips and sometime under the nose or under the chin. Cold sores are usually caused by herpes virus type I and are very contagious.

Canker sores usually heal in about a week or two. Rinsing with antimicrobial mouthrinses may help reduce the irritation. Over-the-counter topical anesthetics can also provide relief. Cold sores usually heal in about a week. Over-the-counter topical anesthetics can provide temporary relief and prescription antiviral drugs may reduce these kinds of viral infections.

(My 2 cents from personal experience: L-lysine is good for keeping cold sores at bay.)