One of the benefits of permanent residency is permission to work. However, permission to work (“work authorization”) is not limited to permanent residents.
Some nonimmigrant statuses (such as tourists and transits) completely prohibit any work, but many foreigners in the U.S. may legally work. Whether they are granted such permission will depend upon the kind of status they had when admitted and whether they have pending applications for a more permanent status.
Other forms of status restrict work to a specific employer and will depend on the visa under which the person was admitted. Others allow for employment, but a specific request for work authorization must be made before the person can begin.
Work authorization is extremely detailed. In some cases, making an application can place one’s status in jeopardy. So see a lawyer who is experienced in immigration law for advice.