U-Bahn and S-Bahn
The Berlin U-Bahn (subway or metro) is something to behold; it is so charmingly precise!
There are no turnstiles to limit access, so it is technically possible to ride without a ticket, but if caught by a ticket checkers you will be fined €40 so it is probably not worth the risk.
All U-Bahn stations now have electronic signs that give the time of the next train, and its direction based on sensors along the lines.
Detailed maps can be found in every U-Bahn station on the trains.
U-Bahn stations can be seen from far by their big, friendly blue U signs.
Together with the S-Bahn, which is administered by Deutsche Bahn and mostly runs aboveground, the U-Bahn provides a transportation network throughout greater Berlin that is extremely efficient and fast.
On weekend (friday to sunday), as well as during the Christmas and New Year holidays, all U-Bahn and S-Bahn lines (except line U4) run all night, so returning from late night outings is easy, especially given the average start time of most 'parties' in Berlin (11 p.m. to 1 a.m.).
During week is no U-Bahn or S-Bahn service from appr. 1 a.m. to 4.30 a.m., but metro trams/buses and special Night Buses run every half an hour from 0.30 to 4.30 a.m.
For a single journey you can buy a cheap Kurzstrecke for €1.20, but this is only valid for 3 stops on the U-Bahn or S-Bahn (six stops by bus or tram). You may not change.
For a longer single journey you must pay €2.10, which is valid for anywhere in zone A & B for two hours after validating.
Note that you may only ride in one direction with one single journey, you are not allowed to go around.