Zweites Deutsches Fernsehen. Means, literally: Second German Television. This is the only dedicated nationwide German public TV station, as the ARD is a channel resp. broadcasting system operated and provided with content by regional stations.
Traditionally, the ZDF has always been a bit more conservative with respect to adapting to new trends in TV programming and content. Their music shows and sitcoms tend to be a bit cheesy, and it's a well-known fact that the ZDF caters more to the middle-aged or elderly population than to younger Germany. For example, they don't produce any kind of "cool" daily soap (anymore). ZDF telefilms and shows aren't as good as ARD ones, but there are notable exceptions, for example the cult show abenteuer forschung (which means "The Adventure of Research"; imagine the title typeset in computer-generated lettering that was cool in 1982) with cult moderator Professor Joachim Bublath or Der letzte Zeuge (The Last Witness), which is somewhat like Quincy in the 21st century. In one episode of the latter show, a woman turns out to have been killed by a hunky boxer exhaling violently into her genitals. No shit.