Beginner hikers sometimes overlook two simple facts about hiking clothing for warmth. First is that hiking clothing is not primarily designed to keep the cold out but to keep the heat in from the only source of heat i.e. yourself.
Second is that when you are wearing hiking clothing for warmth, you are not trying to be as warm as possible but to reach a thermal equilibrium i.e. a state in which your heat production roughly balances your heat loss so that you remain within your comfort zone whether you are on the move, on the move and sweating, sitting still and sitting still and sweating.
To achieve this thermal equilibrium under changing conditions your hiking clothing has to be versatile. Of course you can make adjustments to your hiking clothing by taking off layers when it gets too hot and putting more on when it gets cold but in most cases it's both inconvenient and impractical.
The hiking clothing you use needs to be able to handle all the different situations, whether on the move or sitting still, at the same time. They need to be able keep the still and dry air to insulate you, keeping you warm and when you're moving allow the vapor of your sweat to pass through and escape to keep you cool.
If your hiking clothing absorbs some of the moisture so that when you sweat heavily it remains damp, they lose the ability to keep you warm as it has lost its ability to hold dry air which acts as an insulating layer and at the same time your body will continue to draw latent heat from your body needed to turn the sweat into vapor.
The result is that you suffer from what is called the after-exercise chill. Experiments have shown that with an absorptive material the chill can last for about two hours but with a non-absorptive material like fibre-pile the chill is negligible.
The Three-Layer Hiking Clothing System
The ability to prevent after-exercise chill so well is what makes the three-layer hiking clothing system so popular. It's a proven way to ensure your comfort outdoors. Each of the three layers have different functions that work together to cope with the different conditions.
The First Layer
Moisture management is the main function of the first layer. Some experts call it the next-to-the skin layer. It keeps you cool in the summer and keeps you warm in winter by keeping you dry.
Made of some sort of polypropylene or other synthetic fabrics, the fabric 'wicks' away the moisture from the skin very rapidly by capillary action dispersing it to the outer surface where it can evaporate.
The Second Layer
The second layer or middle layer of hiking clothing is the insulating layer. Their main function is to trap air, which is a good heat insulator, so you retain warmth.
This layer is made of natural fibers or fleece. Both are reliable in keeping you warm but natural fibers have a more efficient warmth-to-weight ratio and are highly compressible. Its drawback is that it has to be kept dry to maintain its ability to retain warmth.
Hiking clothing made from fleece are lightweight, breathable and insulate even when wet. They also dry faster and the warmth-to-weight ratio is even higher than wool. Fleece is very popular with many experienced backpackers I know of even when it is not as compressible as natural fibers.
The Third Layer
The third layer is the waterproof shell that both acts as a windbreaker and a rain jacket. If the outer layer doesn't do what it's supposed you'll end up losing a lot of heat as the wind and penetrate into the inner layers.
It is usually made of Gore-Tex that also offers breathability. This layer of hiking clothing needs to have proper ventilation or else perspiration can't evaporate but instead condenses on the inside of your shell.
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