Burns are classified according to the depth of the burn and the extent of surface area involved. On the basis of depth, burns are either partial-thickness or full-thickness burns.

Partial-thickness burns are divided into first-and second-degree burns. First-degree burns involve only the epidermis and are red and painful, and slight edema (swelling) may be present. They can be caused by sunburn or brief exposure to hot or cold objects, and they heal in a week or so without scarring.

Second-degree burns damage the epidermis and the dermis. If there is minimal dermal damage, symptoms include redness, pain, edema, and blisters. Healing takes about 2 weeks, and there is no scarring. If the burn goes deep into the dermis, however, the wound appears red, tan, or white, may take several months to heal, and might scar. In all second-degree burns the epidermis regenerates from epithelial tissue in hair follicles and sweat glands, as well as from the edges of the wound.

Full-thickness burns are also termed third-degree burns. The epidermis and the dermis are completely destroyed, and deeper tissue may also be involved. Third-degree burns are often surrounded by first- and second-degree burns. Although the areas that have first- and second-degree burns are painful, the region of third-degree burn is usually painless because of destruction of sensory receptors. Third-degree burns appear white, tan, brown, black, or deep cherry red in color. Skin can regenerate in a third-degree burn only from the edges, and skin grafts are often necessary.

When skin is destroyed by a burn, the normal homeostatic functions of the skin are lost. Treatments for burn patients attempt to maintain homeostasis until the damage to the skin has been repaired. For example, the skin normally prevents fluid loss, and following a burn, bodily fluid loss increases greatly. To prevent possible shock and death, fluids must be replaced at the same rate at which they are lost. The amount of fluid that will be lost can be calculated on the basis of the patient's body weight and the extent of the burn. For an adult, the surface area that is burned can be conveniently estimated by "the rule of nines", in which the body is divided into areas that are about 9% or multiples of 9% of the total body surface. For younger patients, surface area relationships are different. For example, in an infant the head and neck are 21% of total surface area, whereas in an adult they at 9%. For burn victims under 15 years of age, tables specifically developed for this age group should be consulted.

The skin normally maintains homeostasis by preventing the entry of microorganisms. Burns disrupt this function and present an opportunity for microorganisms to cause infections. For this reason, burn patients are maintained under aseptic conditions, which prevent the entry of microorganisms into the wound. They are also given antibiotics, which kill microorganisms or suppress their growth.

Deep partial-thickness and full-thickness burns take a long time to heal and form scar tissue with disfiguring and debilitating wound contracture. Skin grafts are performed to prevent these complications and to speed healing. In a split skin graft, the epidermis and part of the dermis are removed from another part of the body and are placed over the burn. Interstitial fluid from the burned area nourishes the graft until it becomes vascularized. Meanwhile, the donor tissue produces new epidermis from epithelial tissue in the hair follicles and sweat glands such as occurs in superficial second-degree burns.

Other types of grafts are possible, and in cases in which a suitable donor site is not practical, artificial skin or grafts from human cadavers or from pigs are used. These techniques are often unsatisfactory because the body's immune system recognizes the graft as a foreign substance and rejects it. A solution to this problem is laboratory-grown skin. A piece of healthy skin from the burn victim is removed and placed in a flask with nutrients and hormones that stimulate rapid growth. The skin that is produced consists only of epidermis and does not contain glands or hair.