(Translated from the German wikipedia article)

The S-Bahn is a suburban metro railway network in Germany. The name is an abbreviation for "Stadtschnellbahn" (fast city train) and was introduced in December 1930 in Berlin, after "SS-Bahn" (Stadtschnell...) had been inofficially in use already. The S-Bahn is a railroad that serves metropolitan traffic as well as direct regional traffic, and is characterized by high efficiency and a synchronized timetable that allows for denser train traffic on the rail lines. This is achieved by the use of separate rails/tracks, electric railcar-driven trains and level-free train doors (in some cities these standards are not met though). The symbol for the S-Bahn in Germany is a white "S" on a green circle. In Switzerland, S-Linien (S-lines) is displayed in black letters on a white background. From the term, S-Bahn, come many similar notations, like the name R-bahn for regional trains, which is not in accordance with S-Bahn criteria.

The term "RER" (Réseau express régional), used in France and western Switzerland, and the "S-tog" in Copenhagen are very similar to the S-Bahn.

Classifications

S-Bahn lines are different from U-Bahn lines in that they have developed out of the classical railroad. To this day the S-Bahn companies are subsidiaries of Deutsche Bahn, the former monopoly. Compared to subway lines, the S-Bahn lines have numerous branches. The lines of different branches operate on one or several stretches together where they create dense train traffic.

In comparison to the regional train, the S-Bahn experiences a higher efficiency, but also shorter distance between train stations.

See also (pages on German Wikipedia)