For all EU citizens as well as citizens of the EEA and Switzerland, the “free movement of workers” applies in Germany. They therefore do not need a special work permit. However, different legal framework conditions apply to the employment of citizens from third countries.
In general, foreigners from third countries may only work if they are in possession of a corresponding residence permit. They are subject to the Residence Act, which regulates immigration (with the exception of nationals of: Korea, New Zealand, Canada, Japan, Australia, Israel, UK, Northern Ireland, USA). They can enter Germany without a visa and apply for a residence permit on site in Germany.