There is often confusion between canker sores and cold sores. In a nutshell, canker sores are painful ulcers, or open sores. Canker sores are not considered to be contagious and are of uncertain origin. Cold sores, on the other hand, are small red blisters that generally affect the mouth and facial areas, but usually appear on the lip and outer edge of the mouth. In contrast to canker sores, cold sores are extremely contagious.