Thin: Thin fabric, ideal for highly aerobic activities in cold conditions.
Medium: Thick fabric, ideal for slow moving activities (such as walking and climbing) in cold conditions.
Wind Resistant: Thick wind proof/wind resistant fabric, ideal for slow moving activities (such as walking and climbing) in cold and very windy conditions - ie Scotland in winter. However, windproof fabrics such as Gore Windstopper and Polartec Windbloc, have a downside in that they restrict hearing slightly, and whilst non membrane fabrics like Polartec Windpro make it easier to hear, they are not as windproof.