At the end of WWI, Kaiser William II abdicated and fled Germany. After his departure, a new government was set up. The New German Government was called the Weimar Republic, named after the town outside Berlin where it was started. The Government started in 1918, two days before Germany surrendered in WWI.
The Weimar Republic lasted from 1918 to 1933, when the Nazis seized power after being voted into office. During that time, Germany faced many problems that the Weimar Republic was unable to solve. It was this instability that contributed to the rise of the Nazis.