A WG or Wohngemeinschaft is an apartment where multiple people live together and share the rent. It’s called a shared apartment or flatshare in English. Living in a WG is cheaper than living alone, so it’s a good way to pay less rent. Most WGs are listed on WG-Gesucht and Kleinanzeigen.
A Zweck-WG is a WG where people live together to save money, not to be friends.