Burns are classified by different degrees depending on the severity of the burn and how far into the skin layers there is damage.
First degree: Only affects the first layer of skin (the epidermis) and causes redness and mild pain. They are the least serious type of burns.
Second degree burns are divided into two types: Superficial partial-thickness burns and deep partial-thickness burns. These involve the second layer of the skin called the dermis. In addition to redness and more severe pain, blisters may develop that ooze. The skin will appear to be blotchy.
Third degree burns, also called a full thickness burn: Severe burns that affect not only the skin layers but the structures below, such as muscles, tendons, nerves, blood vessels and bone. The skin can appear charred and pale. Usually there is little pain as the nerves have been destroyed. These burns must receive medical attention immediately.
https://www.empowher.com/burns/content/burns-types-and-severity