If you seek to work in the United States, you must be authorized to work in the country and you will have to obtain a permit that will grant you permission to work. This applies to the non-immigrants and not to the citizens of the United States and the Green Card holders. And that is because US citizens are generally authorized to work in America and the Green Cards issued to the permanent residents will permit them to work. But the foreign nationals who get into the country on non-immigrant visas cannot work in America unless they are authorized to do so. Hence, if you hold a non-immigrant visa, you must file Form I-765, Application for Employment Authorization, to obtain an employment authorization document that is also known as a work permit.
Moreover, US employers cannot hire anyone who is suitable for the job and they must first check whether the foreign nationals are allowed to work in America before hiring them. There are few different categories of non-immigrants who are eligible for USCIS work permits and you may file Form I-765 for a work permit, if your non-immigrant visa permits you to accept employment, while in America. Similarly, certain non-immigrant visas permit the holders of those visas to work for specific US employers and if you hold one such visa, you may apply for an employment authorization document. If you belong to a particular category and if you hold a specific non-immigrant visa that requires you to obtain a work permit to work temporarily in America, you may file Form I-756 and obtain a work permit.
Similarly, non-immigrants who have filed Form I-485 for adjustment of status also must apply for work authorization and file Form I-765 and obtain work permits before accepting employment in America. However, they may not be required to obtain employment authorization documents after they are granted Green Cards. After obtaining these work permits, non-immigrants can work for US employers but they may not be able to work in the country on a permanent basis and the USCIS work permits are generally valid for one year and are renewable.
You may use your work permit to prove that you are authorized to work in the country for a particular time period. Similarly, you may file Form I-765, if you need to get your lost or mutilated work permit replaced or to renew your employment authorization document. But you cannot apply for renewal, if your existing work permit is still valid and you can file Form I-765 for renewal only if your existing permit will expire within the next four months. You can also file this form online, if you belong to a category that is eligible for online filing.