Canker sores are small ulcers with a white or gray base and a red border.  They appear inside the mouth and are not contagious.  Fatigue, stress, allergies, trauma and intestinal problems can increase the likelihood of a canker sore.  They usually heal on their own after a week or two, but more severe outbreaks may occur.  Over-the-counter topical anesthetics and antimicrobial mouth rinses may provide temporary relief.  Avoid hot, spicy or acidic foods that can irritate the sore.
Cold sores are groups of fluid-filled blisters that can be painful.  They often erupt around the lips and sometimes under the nose or around the chin.  Cold sores are caused by herpes virus type 1 and are very contagious.  Once a person is infected, the virus stays in the body and causes occasional attacks.  These attacks may follow a fever, sunburn, skin abrasions or emotional upset.  Most blisters heal in a week by themselves.  Early treatment of prescription antiviral drugs may reduce these kinds of viral infections.