Keeping dry is actually the number one challenge that campers deal with, not the temperature. In warmer weather, besides blisters on your feet, moisture isn't as critical as issue. However, as the temperature drops, moisture plays an increasing part in your comfort and even saftey.

Moisture doesn't just fall from the sky or soak up from the ground. Your body is constantly evapourating water and this moisture gets into your clothes. You learned in your science class that liquids want to change into a gas. This takes energy. The energy to do this is pulled from your body. If you have too much energy being pulled from your body then you feel cold and if you can't replace your energy faster than you loose it, then a condition for Hypothermia exists.

Some materials have different properties in reacting to mositure. Cotton looses almost all of its insulating properties when wet. This is why that cotton sweatshirt after you have been wearing it all day isnt as warm as it was a few hours ago. Same goes for the cotton socks you are wearing in your boots. Cotton also tends to stay wet when wet. This is the reasoning behind the recommendation when posible to avoid cotton.

Synthetics on the other hand, do not have the same properties as cotton. Popular for their wicking qualities (drawing water away from your skin) synthetic fibers dry quickly and help keep your body warm in when it's cold out. For sweaters, fleece materials are ideal light weight modern substitutes. They hardly soak any moisture at all, and if they do get damp they dry very quickly. Wool still has its place, it maintains 80% of its insulating properties, even when wet. But pound for pound, the outdoor fleeces on the market will outperform wool when it comes to bulk. Synthetic socks year round will help keep your feet comfortable. Cotton socks tend to hold moisture in the feet. In the summer they will feel more hot and sweaty and the moisture in your feet can cause blisters on the trail. In the winter, heavy wool socks in the winter are warmer than anything I have tried.

(A similar topic on keeping dry can be found in the 'Canoeing' section on 'Waterproofing Your Gear' linked on the homepage.)