The Reichstag is the building that houses the German Parliament, the Bundestag. It was severely damaged during the Second World War, and was not used until German reunification. It was in West Berlin, but next to the Berlin Wall and hence not an ideal place for the West Berlin Government.
After German reunification in 1990, it underwent reconstruction that was by the British architect Norma Foster. After reconstruction, it once again became the meeting place for the German Parliament.
Norman Foster’s design included a glass dome at the top of the building. This dome was to signify Government transparency. From the dome, you can look down into the chamber of the parliament.
The dome is open to the public and is free to visit. Given it is a the German parliament, you do have to book tickets before hand, turn up with your passport and go through metal detectors. It is well worth the visit.
More information on the Reichstag Dome