Canker sores, not to be mistaken with cold sores, are small sores that appear on the inside of the mouth or at the bottom of the gums. These sores are actually pretty common, affecting about 3 million people in the United States per year.  Although often mistaken for cold sores because of their similar appearance, canker sores are very different in that the sores, or blisters do not occur on the lips and are not contagious like in cold sores.  It is important to remember that cold sores are a result of herpes virus infection, while the causes of canker sores are unclear and variable.