As the next-to-skin layer, a base layer moves perspiration away from your body, this is  known as "moisture wicking". Base layer fabrics make great t-shirts and tank tops in warmer weather as well as the first layer of your layering system in colder weather.  You need to keep your skin dry in cold conditions to avoid becoming chilled, or even hypothermic. In warmer weather, they keep you dryer for comfort. When layering, a base layer should fit snuggly so a fabric with adequate stretch is important.  Base layer fabrics may be lightweight and thin or a little thicker/fluffier, if you prefer, in colder weather.  The main function of a base layer is to move moisture away from your body.  We have a couple of blog articles that may be helpful as well:  Layering 101 and Packing for Multiple Climates