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. Currently headed by Chancellor Angela Merkel, the CDU has around 431,900 members (as of 2016).
The CSU is considerably smaller (around 142,400 members as of 2016) and headed by Horst Seehofer. Its colour is light blue. 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.