Fever blisters or cold sores, are caused by the herpes simplex virus, and are commonly mistaken as canker sores. The difference between a cold sore and a canker sore (apthous ulcer) lies in their cause and location.
Cold sores are caused by the virus Herpes simplex and may occur inside or outside the mouth, as well as on the chin or in the nostrils, and they often come back in the same location. They can be spread by mouth-to-mouth contact, with kissing being the most likely cause of spread.
Canker sores are not caused by a virus and are not spread by kissing and only develop on the inside of the mouth. While both fever blisters and canker sores might look the same with a reddish and inflamed appearance, canker sores might also appear white and usually do not bleed.