Berlin has a very good public transportation system, with a subway (U-Bahn), suburban trains (S-Bahn), street trams, and buses. As a student, you can purchase a semester ticket, which is valid for the current semester and which allows you to use the public transportation in Berlin.
Most students from Berlin’s universities and Fachhochschulen obtain their personal semester ticket when they register for their courses. The semester ticket can only be used by the person it was issued for and is only valid in conjunction with your student ID. You must purchase a new ticket at the start of every semester. The price of the ticket is included in the registration fee (Immatrikulationsgebühr).
There is the possibility, on certain conditions, of having the semester ticket costs fully or partially refunded back to you. You can get further information from the Semester Ticket Office of your university or Fachhochschule.