CDU/CSU, unofficially also referred to as the Union parties or the Union, is a term referring to Germany's two main conservative parties, the Christian Democratic Union of Germany (CDU) and the Christian Social Union of Bavaria (CSU). They are considered sister parties, since they cooperate on multiple issues. The CSU is only organised in the German state of Bavaria and only fields candidates in elections within Bavaria (in German Bundestag and European Parliament elections, it only runs for Bavarian mandates), while the CDU correspondingly has only subsidiaries outside of Bavaria and only runs in the other 15 states. Both parties' representatives in the Bundestag share a common faction, called the CDU/CSU faction or unofficially union faction, both parties are members of the European People's Party, and have a common youth organisation called the Young Union. Both parties nonetheless remain legally separate parties with distinct apparatus, membership, funding etc.
Read more about CDU/CSU: Political Stance, History, Tensions, Parliamentary Chairpersons of The CDU/CSU Group in The Bundestag