The RWTH is the umbrella organization for nine faculties, which offer in total 115 courses of study. The faculties are responsible for the organization of the courses. The faculties decide about the content, the lectures and the structure of their courses of study. So if you have any organizational question, it is your faculty that you need to address.
Every faculty has numerous institutes connected to it. In total there are 260 of them. Some of those institutes only consist of very few people and offer only one lecture; other institutes are like huge companies with over a hundred employees, who conduct research and work in projects for the industry. The institutes are those who do the actual work at our university. That means, that they have the employees who write you books, create, correct and mark your exams and hold your lectures. So if you have any specialist question about a topic in the lecture, it is the responsible institute that you need to address. Every institute is led by one or more professors. The professors, who hold your lectures at the RWTH are not just teaching, but they are mainly active scientist in their field and they are leading their own team of researchers.
Other important organizations are the AsTA “Allgemeiner Studierenden Ausschuss” = Student Union and the “Fachschaft” the Union of all those Students in one study course. If you have any questions about study, about work experience or about life in Aachen, if you have been treated unfair by a professor or other university staff or if you have any other kind of bureaucratic trouble do not hesitate to ask them for help. Simply search in google for “RWTH Fachschaft + your study course”