Berlin boasts quite a spiffing public transport system. It’s run by the yellow BVG, and using it is easy enough. However, visits from friends and family have taught me about a number of frequent pitfalls for the occasional rider.
Public transport here is made up of the U-Bahn (underground), S-Bahn (suburban trains), and trams and buses.
The most important rule to observe: always stamp your ticket (including the visitor tickets mentioned below), unless you buy one from machines on the tram, where it comes pre-stamped with the date and time. The stamping machines are red boxes on train platforms and yellow boxes on trams and buses. Unstamped tickets will be considered invalid! The BVG is doing its best to catch fare-dodgers in the act and doesn’t make any concessions to tourists not familiar with the stamping concept.