Layering simply means wearing a combination of clothes (in layers) to help regulate your temperature, and keep yourself warm and dry. The layers you wear for a given activity are matched to the weather, your activity level and personal preference
There are essentially three layers to consider: base, mid, and outer. Each layer has a specific function. The base layer wicks moisture & perspiration away from your skin, which keeps you dry; the mid layer is for insulation, which keeps you warm; the outer layer allows moisture to escape from the inside while blocking wind and repelling water from the outside, which completes the layering system. You'll be moving in your own private micro-climate, and comfortable out in the elements.
The Base Layer is in contact with your skin. A snug-fitting (but non-restrictive) wicking material is best to keep you warm and dry. Polypropylene, silk, polyester, Thermax, Thinsulate, and merino wool are all good choices Avoid cotton because it traps moisture, so it stays wet and draws heat away from you. Base layers come in various weights (lightweight, midweight and heavyweight). Select a weight based upon the outside temperature and your activity level. The lighter weight is better at wicking, the heavyweight has more insulation.
The Mid Layer provides insulation. It should be a bit looser than the base layer, but to function properly it needs to maintain contact with the base layer. Mid layers also carry moisture away from the base layer to the outer layer, so it needs to be a conductive fabric. Common material for mid layers include fleece, wool, down, and newer synthetic/natural blends. Mid-layer clothing performance extras include pit zips, long front zippers, adjustable cuffs and collars.
The Outer Layer blocks wind and allows moisture to escape. Typical outer layers include shells made of waterproof/breathable membranes, newer "softshell" laminates or a similar material. Extras such as pit zips, ankle zippers (for pants), hoods (for jackets) and a variety of ventilation options are standard. Outer layers should also be tough enough to withstand tears and abrasions. Other less high tech options may include wind resistant materials, or water resistant fabrics.
Once you have a layering plan, you can adjust your temperature control simply by removing or adding layers as needed.
Another tip for maintaining the life of your clothing is to follow the care instructions on the item. Synthetic, technical fabrics need to be cleaned properly in order to wick, insulate and repel water. They should be washed on "gentle" cycle, and be air-dryed whenever possible. Special cleaning products can restore the moisture wicking and water repellent properties if necessary.
Head, Hands and Feet
After your core is covered, you need to properly dress your extremities. Wear a hat, mittens /gloves, socks and shoes that match your activity and weather conditions. To cool yourself if you overheat, you can often just remove your hat or gloves. Keep in mind that wind blocking fabric is also important for hats and gloves. Although fleece is warm, it usually does not provide protection from wind.
Proper layering will not only make you more comfortable during winter activity, but it also keeps you functioning safely.
Have fun out there!