base layer is exactly that- the base level of clothing. It is designed to wick moisture away from your skin and out to the next layer of clothing that you wear on top of it. My cold weather gear would usually consist of a base layer, then a long sleeve cycling jersey, then a jacket- something to stop the wind, at least in front. On the bottom, I have a pair of windstopper tights. As the weather gets warmer, I dump the long sleeve jersey and tights in favor of a short sleeve jersey and shorts with arm and leg warmers, so I can remove layers more easily if the day gets warmer than I expect. I also have sleeveless base layers for the same reason.