They don't need to be anything fancy, as long as they're taller than your boot cuff and NOT 100% cotton. The height is obvious and has already been explained. Cotton absorbs moister, and, contrary to popular belief, doesn't breath well. Cold nor not, your feet will sweat when you ski. With cotton, sweat is absorbed by the sock rather than transferred to the liner causing the fibers to expand. The sock will no longer fit correctly and will tend to bunch up, creating pressure points.
With a different material the sock won't expand, it will stay fitted to your foot, your skin will be able to breathe and you will be much more comfortable. Polyester-spandex blends are a good choice as they transfer moisture well, retain little of it and have good elasticity.
Hope that helps.