Burns are categorized into first, second, or third degree, and depends on how deep and severe they penetrate the skin’s surface.

First degree burns (Superficial)
The burn only effect the epidermis (outer layer of skin). The burn site can be described as red, painful, dry and no blisters. Long term tissue damage is rare and usually consists of an increase or decrease of the skin colour. Usually heal within 7 to 10 days without scarring.
Eg: Sunburn, Flash burn

Second degree burns (Partial thickness)
The burn effect the epidermis and part of the dermis. The burn site can be described as red, blistered and may be swollen and painful. Some blisters pop open, making the burn a wet or weeping appearance. Usually heal within two to three weeks without scarring but changes the skin pigment.
Eg: Sunburn, flames, skin that briefly comes in contact with a hot object

Third degree burns (Full thickness)
The burns effect and destroy the epidermis and dermis. They also can cause damage to the underlying bones, muscles and tendons. The burn site can be described as white or charred appearances, no sensation in the burnt area as the nerve endings are totally destroyed. Without surgery, these injuries heal with severe scarring and contracture. There is no specific timeline for complete spontaneous healing for third-degree burns.
Eg: Flames from fire, chemical source, scalding liquid

Fourth degree burns
The burns include all of the symptoms of a third degree burn and also extend beyond the skin into tendons and bones.

Burns affecting 10 percent of a child’s body and those affecting 15 to 20 percent of an adult’s body is debated to be major injuries and call for hospitalization and extended rehabilitation.

http://www.healthline.com/health/burns?m=0#complications7
https://stanfordhealthcare.org/medical-conditions/skin-hair-and-nails/burns/stages.html