U-Bahn & S-Bahn Rapid Transit

The easiest, most efficient way to get around
Word to the wise: Tickets must be validated at one of the station or platform machines before stepping onto the train.

The S-Bahn and U-Bahn are rapid transit services which are essentially the same thing, just run by different companies. All tickets are usable on both services, as they are for additional journeys on the buses and trams. Basically, one travel ticket will let you travel by any means for the following two hours in one direction.
The S-Bahn runs mainly overground and stretches as far as Brandbenburg and Potsdam, whilst the U-Bahn covers the majority of underground lines. There are three zones: A, B and C and three different tickets - AB, BC and ABC, with AB covering all of the city centre.

Trains run until around 1am weekdays and all night on weekends, and you're never usually waiting more than five minutes for one to arrive. Plus, they're clean, comfortable and amazingly punctual. Tickets should be purchased before travel, from station forecourts, vending machines, and many retail outlets, or from the BVG (Berlin state transport) webpage.