Your base layer of clothing is the first layer clothing worn closest to your skin. This layer will change from one environment to the next. Sometimes its primary purpose is to keep you warm, other times to keep you cool, but it should always wick moisture away from your skin. In cold weather moisture trapped against your skin can rob your body of heat and can eventually lead to hypothermia. In hot weather, a moisture-wicking layer can facilitate you body's natural cooling process by pulling moisture away from the skin and allowing it to evaporate.
Generally made of either cotton or synthetic, a T-Shirt is there to help keep you cool and dry by wicking away moisture from you skin.
Underwear is very important because the areas of the body covered by underwear can produce a significant amount of moisture.
In cold conditions you will want a Thermal Layer to keep you warm. This layer can be worn on top of a t-shirt.