Students from the European Union, Iceland, Liechtenstein, Norway and Switzerland have free access to the German labor market and are practically equal to German students. However, if they work more than 20 hours per week, they (as well as German students) have to pay certain insurances.
There are special legal rules pertaining to students from other countries:
Students: If you are resident in Bremen and possess a visa, you can work 120 days (or 240 half days) in a year; as it says on your visa.
This does not apply to students who are still preparing for the German language exam (e.g. TestDaF) or attending courses of preparatory studies (Studienkolleg). For them, the rules are stricter. You may work during the holidays only with the consent of the Labor Office and the Immigration Office. In the second year of preparatory studies, gainful employment may be exercised to the same extent as regular students.
Graduates: Once you have successfully completed your studies, you have 18 months to find a suitable job in Germany. The 18-month period starts from the date on your graduation certificate! During this time, you may also pursue an activity that does not correspond to your degree.
Further information on the minimum wage, taxes and insurance can be found on the web pages of the German Academic Exchange Service - DAAD.