Skin forms the largest organ of the body – accounting for about 16 per cent of a person’s weight.
It performs many vital roles as both a barrier and a regulating influence between the outside world and the controlled environment within our bodies.
Internal body temperature (core temperature) is controlled through several processes, including the combined actions of sweat production and the rate of blood flowing through the network of blood vessels within the skin.
In cold conditions, skin blood flow drops very low – retaining heat in the centre of the body and preserving the flow of warm blood to the vital organs within the chest and abdomen and to the brain.
Skin and the fat layer beneath it also act as good insulators. When in a hot environment, or when generating heat from muscle activity, skin vessels open up – potentially increasing skin blood flow to as much as a third of the total output of blood from the heart.
Therefore the skin turns into a heat radiator. The evaporation of sweat from the skin greatly increases the rate at which heat can be lost from the body.
The physical toughness of the skin prevents the ingress of harmful chemicals and invading organisms, such as bacteria and viruses. It also provides resistance to shocks for the more sensitive tissues underneath. At the same time, skin needs to be supple and be able to stretch to accommodate movement.
Skin exposed to sunlight is the main site of manufacture of vitamin D, which is essential for the growth and maintenance of our bones.
The extensive network of nerves, within the skin, feeds information constantly to the brain concerning our surroundings. On the one hand we are warned of harmful extremes of temperature or of other dangers – while on the other, touch can be one of the most powerfully soothing and pleasurable of sensations.
Much importance is attached to the appearance of skin, especially in our modern society.
Medical conditions affecting the skin can have marked effects, not only on our state of well being but also on the ways we interact with other people, on our suitability for certain occupations and on the sorts of pastimes we can enjoy.
The layers of skin
There are two main layers of skin:

• epidermis
• Dermis.

• Melanocytes
  These cells produce a dark pigment called melanin which contributes to skin color and provides UV protection. They are located at the bottom of the epidermis.

• Dendritic (Langerhans) cells
  These cells are involved in the epidermal immune system. They engulf foreign material that invades the epidermis and migrate out of the skin to stimulate an immune response.

• Basal cells
  Small cells found at the bottom of the epidermis. Earlier it was believed that basal cell carcinoma is derived from these cells. As of this writing basal cell carcinoma is thought to arise from non-differentiated cells from the basal cell layer.

Dermis
The dermis consists mostly of connective tissue and is much thicker than the epidermis. It is responsible for the skin's pliability and mechanical resistance and is also involved in the regulation of the body temperature. The dermis supplies the avascular epidermis with nutrients by means of its vascular network. It contains sense organs for touch, pressure, pain and temperature (Meissner´s corpuscles, Pacinian corpuscles, free nerve endings), as well as blood vessels, nerve fibres, sebaceous and sweat glands and hair follicles.

• Blood Vessels
  These are tiny pipes through which blood circulates. The blood vessels supply the skin with fresh blood, which contains nutrients and oxygen, and carry away waste products.
• Meissner's corpuscle
  These touch receptors are expecially effective in detecting light touch and soft, fleeting movements.
• Pacinian corpuscles
  Pacinian corpuscles function as receptors for deep pressure and vibration.
• Free Nerve Endings
  Free nerve endings are sensitive to pain, temperature changes and itchiness.
• Nerve Fibers
  Nerve fibres forward information.
• Sebaceous Glands
  Sebaceous or oil glands are small, sacculated organs that secrete sebum. This oily substance is a natural moisturiser which conditions the hair and skin. Sebaceos glands are found all over the body, but they are more numerous in the scalp area and around the forehead, chin, cheeks and nose.
• Sweat Glands
  These are sweat-producing structures consisting of a single tube, a coiled body and a superficial duct. They are involved in thermoregulation, as they cool the skin by sweating.
• Hair Follicles
  Hair follicles are downward growths into the dermis of epidermal tissue and produce hair. They are found all over the body except on the palms of the hands and soles of the feet as well as on the lips. When the body gets cold, the hair stands upright with the help of the arrector pili muscle, closing up the skin's pores and keeping the warmth in.
• Arrector pili muscle
  This small muscle is attached to the base of the follicle. When it is stimulated by cold or fright, it pulls the hair follicle up, causing it to stand upright.

Subcutaneous layer
The subcutaneous layer below the dermis consists of loose connective tissue and much fat. It acts as a protective cushion and helps to insulate the body by monitoring heat gain and heat loss. Not all authors consider this layer a part of the skin, but it definitely has a strong impact on the way the skin looks.
Knowing your skin type is the chief way you can ensure your skin feels and looks its best. Many people, however, get confused when trying to figure out exactly what their skin type is. Skin can be divided into various types according to its texture. It is essential to know what skin type you actually have and using the correct products on your face can really make a difference to those problem areas you may have.
Due to our higher levels of collagen, male skin is up to 20% thicker and tougher than female skin. Male skin is also oilier, with larger exposed pores, a richer blood supply, and more of a tendency to break out in a sweat compared with women's skin. There are five basic skin types. They are 

• Normal skin
• Dry skin
• Oily skin
• Combination skin
• Sensitive skin

Normal Skin
Normal skin has a fine even texture with a supple and smooth surface. It has a proper balance between oil and moisture contents and is therefore moist and is neither greasy nor dry. The pores of normal skin are small and the overall skin tone is even. It looks clear and does not develop spots and blemishes. The pores of the skin are fine and barely visible. It reflects good health and needs gentle treatment.
Dry skin
Dry skin has a dry parched appearance and has a tendency to flake easily. It is prone to wrinkles and lines. This is due to the inability to retain moisture as well as the insufficient production of sebum by the sebaceous glands. Dry skin often has problems in cold weather and ages faster than normal or oily skin. Constant protection is very important. It needs a moisturizer during the day and a good cream at night.
Oily skin
As its name implies, oily skin is slightly to moderately greasy. This type of skin is caused by the over secretion of sebum making the skin surface oily. The excess oil on the surface of  the skin attracts dirt and dust from the environment. Oily skin is also prone to black heads, white heads, spots, pimples and such skin will never be clear. This type of skin needs to be cleansed thoroughly.
Combination skin
This type of skin is very common. As the name suggests, it is a combination of both oily and dry skin with certain areas of the face oily and the rest dry. Usually there is a central greasy panel consisting of the forehead, nose and chin and a dry panel consisting of cheeks and the areas around the eyes and mouth. The greasy central panel is referred to as T - Zone. In such cases, each part of the face should be treated accordingly - the dry areas as for dry skin and the central panel as for oily skin and use skin care products made especially for combination skin.
Sensitive skin
This type of skin has a fine texture and is very sensitive to changes in the climate. This skin type show the disadvantages of  bleaching, waxing, threading, perfumes,  temperature extremes, soap etc and for men sun, perfumes, shaving cream etc. People with this skin type should avoid products with dyes, perfumes, or unnecessary chemical ingredients that may irritate the skin.