Every US employer must check that all employees, regardless of citizenship or national origin, are authorized to work in the USA. Certain temporary aliens in the US can apply for employment authorization (work permit) by filing the form I-765, along with required documents.
Employment Authorization Document (EAD) is a document issued by US CIS that serves as evidence that the holder is authorized to work in the United States.
If you are a US citizen, lawful permanent resident (green card holder) or a conditional resident, you do not required EAD to work in the US. Proof of you US citizenship or green card is your proof of work authorization in the US. If you are authorized to work for a specific employer, such as foreign government, you do not need EAD. Your passport and your form I-94 Arrival/Departure Record (“white card”) serves as your authorization to work in the USA.