Layered clothing

Layered clothing is a term describing a way of dressing using many garments that are worn on top of each other. Some of the layers have different, largely non-overlapping, functions. Using more or fewer layers, or replacing one layer but not others, allows for flexible clothing to match the needs of each situation. Two thin layers can be warmer yet lighter than one thick layer, because the air trapped between layers serves as thermal insulation.
Layered clothing is particularly relevant in cold climates, where clothing must at the same time transfer moisture, provide warmth, and protect from wind and rain. In a hot and dry climate, clothes have very different functional requirements: they must block the radiation from the sun, and allow for sufficient air circulation. Therefore, layered clothing in the sense used in this article is largely irrelevant in hot and dry climates. The wicking layer moves the sweat from your skin, where it can be absorbed by the other layers.
Outdoor and sports wear manufacturers favor layered clothing because, among other reasons, it allows them to offer so-called "technical" or "functional" clothes which are optimized for the particular demands of a specific sport or activity. Such clothes are often made of advanced synthetic materials or wool, and is not very expensive
Layers.
Usually at least three layers are identified as follows:
Often clothes combine two adjacent layers, as in the case of warm undergarments that provide both comfort and insulation.
Inner or base layer.
The purpose of the inner layer is to draw the sweat away from the skin to the next layers, which makes the wearer feel warmer and more comfortable. The transfer of moisture happens due to capillary action, sometimes called wicking. The used materials are called "wicking materials". When moisture has moved from the skin into (nonabsorbent) clothing, it has more surface area and will evaporate faster. If a piece of clothing does not transfer moisture well, it is not strictly an inner layer garment at all, but simply a comfortable mid-layer garment.
Mid layer.
The mid layer is needed in cold weather to provide additional insulation. The use of multiple thin layers facilitates adjustment of warmth. The mid layer should be more loose-fitting than the inner layer, as this leaves insulating air between the layers. However, if best possible moisture transfer is desired, too great a gap between any adjacent layers of clothing may reduce the moisture transfer by capillary action from one piece of clothing to another. On the other hand, very loose-fitting layers can allow more removal of moisture (and heat) via air circulation.
Shell layer.
The outermost clothes are called the shell layer, but only if they block wind or water, or have good mechanical strength. Ideally the shell layer lets moisture through to the outside (that is, is breathable), while not letting wind and water pass through from the outside to the inside. While this is enabled to some degree by modern materials, even the best and most expensive materials involve a slight trade-off between breathability and water- and wind resistance.
If heavy sweating is expected, one should avoid wearing any shell layer garments unless their protective properties are essential. For example, when one is jogging, no traditional shell layer is likely to be able to transfer enough moisture to keep the wearer feeling dry. But as more air permeable membranes emerge, when combined with pit zips the amount of moisture being transferred outwards would be sufficient for cardiovascular pursuits. As a general rule, one should consider using sufficiently warm mid layer clothes.
Both "soft" and "hard" shell jackets and layers exist. Hard shells are commonly woven fabric and do not rip. Soft shell may rip easier, but are more flexible. 
Adjusting a layering system.
As the intensity of exercise or environmental conditions change, the amount or quality of layers worn should be changed. In particular, if clothes become wet from sweating during heavy exercise, they can be much too cold during the following period of rest. Below are two basic strategies for this problem.
Vents.
Some clothes feature adjustable vents, such as below armpits. The positioning and design of the vents allow for area specific cooling while only marginally reducing exterior moisture resistance.
Removing layers.
Removing the shell layer during heavy exercise can pose problems if the clothes underneath lack required mechanical strength or waterproofing. In such situations, using a mid layer with enough of the required shell layer properties can be a good choice.
Fashion use.
Combining different garments in layers can be used to create a variety of outfits. This provides similar practical benefits for practical layering, in that the wearer can shed layers according to changes in temperature, and is also a way of making use of clothing to produce different looks and mix colors in various ways. It saves money for the consumer, who can create a totally new look simply by swapping out one piece of the outfit.