A lager is a bottom fermented beer. By this we mean that they are fermented using bottom-flocculating yeasts that work at very low temperatures over an extended period of time. Following fermentation, the yeast settles to the bottom. Some specialty lagers include pilsner, marzen, and bock. Ales are generally more complex than lagers.
Different kegs have different tap requirements.
European kegs, for example, differ from most U.S. brewed beer. In the U.S., there are even various styles. A small start up brewery may choose an inexpensive tap system to get started, and move up to a different one when they have more capital.