Mouth sores come in a variety of forms. The most common are:

• Cold sores, also known as fever blisters. These are fluid-filled blisters that burst and develop an outer crust. They usually appear on or around the lips, but sometimes may appear inside the mouth. The blisters themselves can be painful, and are usually preceded by a tingling or burning sensation (called the prodrome stage). Cold sores are contagious and can be transmitted through direct contact with infected areas or secretions, including saliva. They may reappear during periods of illness (e.g., colds, fever), stress or after sun exposure in people who have previously had cold sores. Cold sores can also be transmitted during mouth to genital contact during a cold sore eruption.
  
  
• Canker sores. These painful open lesions (ulcers) usually appear in the soft tissue inside the mouth – on the cheeks, tongue or at the base of the gums. Unlike cold sores, canker sores are not contagious, although they may recur in people who have had them. They are usually preceded by a tingling or burning sensation, and typically appear as pale or yellow sores with a red ring around each sore. Canker sores are also referred to aphthous stomatitis or recurrent mouth ulcers.
  
  
• Discolored patches of skin. These may occur due to thrush or leukoplakia. Thrush is a yeast infection that occurs in the mouth, signaled by white patches in the corners of the mouth, on the inside cheeks, tongue, roof of the mouth (palate) and throat. Attempts to remove the white patches may reveal red, inflamed skin that is likely to bleed. The condition is not usually painful.
  
  Leukoplakia, on the other hand, involves white or gray areas that usually appear on the tongue, although it may also develop on the gums, inside of the cheeks or palate. Affected areas are not usually painful, although they may become sensitive to spicy foods, temperature or touch.

Mouth sores are different from tissue irritation or inflammation commonly seen in cases of gingivitis or periodontitis, both of which are due to the buildup of plaque on the teeth (resulting from poor dental hygiene) and may cause the gums to bleed.