CDU stands for Christian Democratic Union of Germany (“Christlich Demokratische Union Deutschlands”). CSU is short for Christian Social Union (“Christlich-Soziale Union”). CDU and CSU work together in a single group in the German parliament, with CSU operating only in the state of Bavaria and CDU covering the other 15 states of Germany.
Black is known to be the colour of the CDU. It was founded after the Second World War as various conservative German groups and associations came together to shape a unified political party. The CDU, which is classified as a people’s party (“Volkspartei”), attracts voters across all social classes and groups.
The CSU is considerably smaller than CDU. Although many see the two effectively a single political body, CDU and CSU are not on the same page on all the issues, namely CSU's strict positions on immigration. The CSU is known with the colour light blue.
Since the 2017 parliamentary election, CDU and CSU jointly occupy 32,9% of the seats in the “Bundestag”. On the national level, CDU and CSU are categorised as union parties (“Unionsparteien”).