Function for Downhill Skiers
Downhill skiers who enjoy going into the backcountry where there are no lifts use climbing skins to gain traction when skiing up the mountains they wish to ski down. This is more efficient than hiking or post-holing the entire ascent.
Skiing skins can be left on when gliding down minor hills, but are removed for the main, steeper descent--the skins will always create some resistance and slow the speed considerably. Their main purpose is to create grip, not glide.
Skiing skins are made of a short-haired nylon fiber that runs in one direction in order to provide traction when going up a hill. Most skins are designed with a clip that attaches to the front tip of the ski and also a simple clip to attach to the back. The underside usually has a very sticky but removable glue material so the skin sticks snug to the bottom of the ski.
When ascending with skins in extremely steep terrain, they may still slip a little depending on how weight is shifted on them, but most randonee or alpine touring bindings have an adjustment under the heel, a sort of heel rest of several inches, to diminish the extreme angle of the ski boot while going uphill.
Contrast to Wax
Skiing skins are preferable for steep ascents, whereas ski wax is preferable for skiing over gentle hills with Nordic skis. On flatter terrain, wax is usually better than skins, because it will still give you some glide, while skins will not.