THIS NEEDS TO BE REWORDED AS "COMPRESSION GARMENTS". SKINS IS A TRADEMARK.
Skins are tight fitting compression sports wear that are designed to apply a balanced and accurate surface pressure over specific body parts, this triggers an acceleration of blood flow, which in turn increases oxygen delivery to the working muscles to enhance their performance. The circulation improvements also help the body to eliminate Lactic acid and other metabolic wastes. The Skins are also designed to wrap around key muscle groups to reduce muscle movement and focus the direction of muscle movement, this wrapping combined with the compression dramatically reduce muscle vibration, resulting in less soft tissue damage and muscle soreness.
You wear them while you are skiing and that evening to help prevent lactic acid build up. Most people that wear them swear by them and claim they don't seize up the day after like they used to and claim they are the best single purchase they ever made to improve the enjoyment of their ski holidays. You regularly see professional sports people training with them on. An indication of the effectiveness of skins is that, if you have some residual soreness in the morning, this soreness disappears or is noticeably reduced when you put skins on in the morning.
Skins are awesome. They are so good that, if your muscles still hurt in the morning, the hurt almost always disappears after you put your skins on. On the other hand, I now wear undies, skins, thermals and a shell. One learns to keep one's legs crossed.
Most skins have NO thermal properties so should not be used as a replacement to thermals in cold climates. Some manufacturers are producing skins with thermal properties. Under Armour, a US brand, is one.
("Skins" is also a term for the nylon bristles which stick to the underside of AT skis to climb steep hills.)