Skin is a waterproof, flexible, but tough protective covering for your body. It is the body's largest organ, covering the entire outside of the body. It functions as a protective shield keeping out water, insects, heat and cold, sunlight, dirt, and gases. It also keeps in body fluids such as water and blood, hormones, minerals, vitamins, and heat. Additionally, the skin plays an important role in protecting the body from microorganisms. The skin accomplishes this is several ways; Through the continual shedding of the upper layer of the skin (desquamation), through the existing normal flora of the skin, and through the excretion of sebum and sweat which contain fatty acids and lactic acid respectively all of which assist in protecting against unwanted microbes.

Over different parts of the body, the thickness and color of the skin and the number of sweat glands, sebaceous glands, hair follicles, and nerves vary. The top of the head has many hair follicles; the soles of the feet have none. The soles and the palms have much thicker epidermis and keratin layers. The fingertips and toes contain many nerves and are extremely sensitive to touch.

The skin tends to change throughout a person's lifetime. A baby's skin has a much thicker fat layer (subcutaneous) and a much thinner layer of protective keratin. As people age, they lose much of the underlying fat, the dermis and epidermis become thinner, the elastic fibers in the dermis become fragmented, and the skin becomes more wrinkled. The flow of blood in the skin also decreases with age; so damaged skin heals more slowly in older people. Older skin also makes less protective oil, so the skin dries out more easily.

The skin is composed of three layers, the epidermis, the dermis, and the subcutaneous layer. Each layer performs specific tasks.