Original: To complete the requirements for a Diplom, one must pass the examinations for the Hauptstudium (main study period) that follow the Vordiplom, complete one or sometimes more than one Studienarbeit (study research project), and write a Diplomarbeit (diploma thesis).
Simplified: To win the Diplom certificate, a student must pass the tests for the Hauptstudium (main study time) after the Vordiplom, then one or more Studienarbeit (study research projects), and then write a Diplomarbeit (diploma thesis).