Form I-765 is used to file for an Employment Authorization Document (EAD).
Filing an Application for Employment Authorization using form I-765 proves your right to work legally in the United States.
Foreign nationals who are temporarily in the U.S. may request employment authorization document using form I-765 and other individuals who are authorized to work in the U.S. without restrictions must also file form I-765 as evidence of their authorization status to work.
http://www.uscitizenship.info/us-visas/i-765-employment-authorization.jsp