Unless you were speaking of water skiing, which we're not, anyone could understand why it might be necessary to have special clothing on a ski trip. Ski clothing has to first and foremost be warm since we're talking about spending the day out in the snow. Secondly, it has to allow a little freedom in movement. Lastly, but not so important to function as form, is ski clothing must be fashionable for the time you spend merely lounging about the lodge.
Warmth is difficult to achieve simultaneously with movement. If you've ever dressed a small child to venture outdoors on a cold winter day, you know that after layering a down filled coat overtop of an under shirt, a turtleneck and a sweater, you have just built a miniature stay puff marshmallow man. Now imagine dressing yourself in ski clothing that creates the same affect and then just try and ski even the bunny hill uneventfully. This is why ski clothing, though designed for layered wear, is still moveable.
Layering is the key to it all. Ski clothing from the outside in is usually ski pants and jacket or a one-piece ski suit with perhaps thermal lined pants or jeans and a sweater or sweatshirt underneath. The outer layer is not only warm and flexible, but also waterproof as well. The body's extremities are covered appropriately as well. A ski mask, as anyone would know is a knit, nylon or fleece blend hood that covers the entire head with the exception of the eyes and the mouth. Gloves are also an important part of ski clothing as warmth is again the key factor with freedom of movement right behind.
When dressing for skiing, most people will layer up and with a decent outer layer, what's underneath will be dry and so long as it's fashionable, will look great while you lounge by the lodge's fire drinking hot chocolate.