“WG” is an abbreviation for “Wohngemeinschaft.” Normally, one flatmate is the main leaseholder of a flat containing multiple rooms. The kitchen and bathroom are shared between all flatmates. In some WGs there is also a communal room. Usually, each flatmate has their own room and pays rent according to its size.
WGs are extremely popular amongst students since they make it possible to live relatively cheaply in trendy, central areas (eg. Neukölln, Kreuzberg, Friedrichshain, Prenzlauer Berg). Furthermore, it is easy to meet other people through one’s flatmates.
In some adverts you will find the expression “Zweck-WG.” This means that the flatmates primarily live in the WG for economic reasons and are not necessarily interested in a social life together.