The Chancellery is only a part of the Bundestag complex spread around the Republic square in central Berlin.
The main and most famous and important edifice of this complex is Reichstag, the German parliament which you can tour (read that review). Reichstag is also the only classic style architecture bldg in the complex, while all others, including the Chancellery are very modern in design - you'd rather think it's a modern art museum or theater than the seat of German Chanellor, currently Mrs. Merkel.
Unlike the Reichstag, you cannot tour the Chancellery - just take few pics in front or from the back if cruising the Spree.