Burns are classified into different degrees based on how deeply and severely they penetrate the skins surface. The higher the degree, the more severe the burn is. The different degrees of burns are:

1. First-degree burns: These burns affect only the outer layer of skin, called the epidermis. The burn site is red, painful, dry, and has no blisters. A mild sunburn is an example of a first-degree burn. Long-term tissue damage is rare.

2. Second-degree burns: These burns involve the epidermis and part of the lower layer of skin, called the dermis. The burn site looks red, blistered, and may be swollen and painful. Second-degree burns can cause lasting damage to the skin and require long-term medical care.

3. Third-degree burns: These burns destroy the epidermis and dermis. They may go into the innermost layer of skin, the subcutaneous tissue. The burn site may look white or blackened and charred. Third-degree burns can cause lasting damage to the skin, muscles, and bones and require long-term medical care. They are the most severe type of burn.

4. Fourth-degree burns: These burns go through both layers of the skin and underlying tissue as well as deeper tissue, possibly involving muscle and bone. There is no feeling in the area since the nerve endings are destroyed. Fourth-degree burns are the deepest and most severe of burns and are potentially life-threatening.

In summary, fourth-degree burns are the worst degree of burn.