Short Answer:
Sores that form around the mouth and lips.
Long Answer:
Cold sores are groups of small blisters that form on the lips and around the mouth. The skin around the blisters can be red, swollen, and sore. The blisters may break open, leak a clear fluid, and then scab over after a few days. They usually heal after several days to 2 weeks, and are also commonly referred to as fever blisters.

Short Answer:
Oral herpes or cold sores are usually caused by HSV-1.
Long Answer:
Cold sores are caused by the herpes simplex virus. You’ve probably heard of herpes before, although, there are actually two types of herpes simplex virus, “type 1″ and “type 2.” Herpes simplex virus type 1 (HSV-1) commonly only infects those body tissues that are above the waist, and it is HSV-1 that causes cold sores in most cases.

Short Answer:
Blisters that fill with fluid, break and turn into lesions.
Long Answer:
The first signs of a cold sore forming, is the appearance of several fluid filled blisters. Usually these blisters are very small and they usually form in a cluster that is no larger than about the size of a nickel. In some cases individual blisters merge with others so to form a single larger blister.
Cold sore blisters will rupture soon after they have formed, resulting in a shallow reddish ulceration whose surface becomes gray. This ulcerative phase of cold sore formation and can be its most painful stage. This is also the stage during which a cold sore is most contagious. In some facial areas the ulcer will become dry and scab over. This scabbing formation is often accompanied by an itching or burning sensation. The scab itself will often crack or break, which in turn can cause bleeding.

Short Answer:
A combination of the patient’s medical history and the appearance of the sores will usually be enough for a diagnosis.
Long Answer:
New tests examine cell scrapings from the blister to detect the virus under a microscope and give fast reliable results usually within an hour of the sample reaching the lab. This type of test is also able to distinguish between herpes type 1 and 2.

Short Answer:
Pain or tingling, followed by small, painful, fluid-filled blisters on a raised, red area of your skin, usually around the mouth and lips.
Long Answer:
Signs and symptoms may not start for as long as 20 days after exposure to the herpes simplex virus, and usually last seven to 10 days. The blisters form, break and ooze. Then a yellow crust forms and finally sloughs off to uncover pinkish skin that heals without a scar.
Also having a sore mouth makes eating, drinking, and sleeping uncomfortable. Cold sores can be painful and a fever, a sore throat, swollen lymph nodes in the neck, and drooling, in small children may also occur.