AStA is an acronym you will come across on almost every campus, especially in western Germany. It stands for Allgemeine Studierendenausschuss, or the general student committee. Every year, AStA representatives are generally chosen from the campus Student Parliament, although this varies from state to state and university to university. The role of AStAs is similar to that of Student Unions in the UK or Student Governments in the US, as it serves an executive role in carrying out actions on campus on behalf of the student body.
The main purpose of the AStA is to expand and defend the rights of students. Most notably, they are the group that negotiates the cost and scope of the Semester Ticket, which is the transportation ticket provided to students. Their cultural sub-committees are also responsible for university-wide parties or festivals.
Regardless of whether you're looking information on financial aid, housing, legal assistance, or need someone to negotiate with the university on your behalf, your AStA is a great place to go for help. On many campuses, they also have budgets for short term loans, which is great if you're a few hundred euros short one meeting your monthly budget.
The AStA generally attracts students planning to go into public life. Unlike the non-partisan elections at most US campuses, political parties play an active role in deciding the makeup of the AStA and the student parliament on German campuses, and the university political scene often serves as a farm league for future politicians.
So acquaint yourself with your AStA's office on your future campus. You'll soon learn that those four letters can be very important to your success.