Baselayer is a general term for thermal underwear which, as the name implies, is underclothing designed for cold conditions. Typically such clothing is made of artificial fabrics which are quick to dry and quick to wick away moisture (ie sweat) from your skin. Traditionally such clothing soon acquires a distinct whiff of its own (!), but modern wool underwear has just about conquered this problem, albeit at a slight penalty in terms of thermal efficiency, weight and durability and not to be outdone thermal underwear manufacturers whose products are based on petro-chemicals have responded by adding bacteria killing metals such as silver to their clothing. Nearly all major outdoor manufacturers have a range of baselayer clothing comprising at the very least, Long Sleeved Zip Top, Long Sleeved Crew Top and Tights/Long Johns, and many nowadays do both artificial fabric and wool ranges.
Baselayer clothing is not just thermal underwear, it is also important as an outer (in fact only) layer in alpine glacier travel where it is too hot to wear anything more. Here it is essential to have long sleeved garments than will ward off the intense sun of high mountain areas.
Underwear: Quick wicking pants and bras.
Light Weight Base Layer: Light weight wicking clothing for running and similar activities in warm conditions.
Medium Weight Base Layer: Medium weight thermal underwear for summer alpine/British winter/expedition use.
Warm Weight Base Layer: Warmer weight thermal underwear for summer alpine/British winter use.
Very Warm Weight Base Layer: Very warm thermal underwear for very cold areas.
Long Johns: Quick wicking thermal leggings for cold conditions, winter climbing and expedition use.