Pilsner is a variety of lager. Other varieties include bock, dunkel, and Oktoberfest/Maerzen.
Lagers are made using bottom fermenting yeast at relatively cool temperatures over relatively long periods of time, while ales are made using top fermenting yeast at warmer temperatures over less time.
For a good and entertaining little history of pilsner, see http:
//beer.about.com/od/lager/a/PilsnerHistory.htm