A WG or Wohngemeinschaft is an apartment where multiple tenants share the rent. It's called a flatshare in English. Living in a WG is a good way to save on rent. Most WGs are listed on WG-Gesucht and eBay Kleinanzeigen. For more details, see this guide.

A Zweck-WG is a WG where people live together to save money, not to be friends.