German. Short for Schnell-Bahn, which means "fast train". This is a system of surface trains in every major city in Germany that provides easy access to nearly every part of the city and its surrounding area. The trains run on tight, regular schedules; the only trouble is that they often don't run at night.
Note that just because the S-Bahn is a surface rail system and the U-Bahn is a subway system, that doesn't mean one will always be aboveground and the other will always be belowground. Usually all trains outside of the heart of the city will be aboveground, and within the city many S-Bahn trains are actually belowground.
While I haven't had the pleasure of visiting any other German cities to date, Berlin's public transportation system is interesting in that each S-Bahn line, U-Bahn line, and Bahnhof (train station) is a little different. There's a sort of personal touch that makes just riding the train from one place to another interesting.
The German rail system (Deutsche Bahn or DB) in general is very efficient and nearly omnipresent. Of course, they must provide a certain amount of access to every inhabited portion of Germany, even if it's at a loss. They also offer fairly good deals and can accomidate nearly anything, including dogs, bicycles, luggage, strollers, and the handicapped, usually without additional cost.