United States allows US employers to hire foreign workers on temporary and on permanent basis and foreign nationals who seek to work in the United States must obtain work visas and this does not apply to US citizens and Green Card holders. Similarly, the country grants visas to various categories of workers that include, researchers, investors, agricultural workers and scientists and such foreign workers must obtain work visas and permission to work lawfully in the United States. Nevertheless, the eligibility requirements differ from category to category and the foreign workers who get into the country on work visas must abide by the laws of the country and must stick on to the terms and conditions of their visas. If not they may not be permitted to work in the country and will be sent back to their home countries.
If you obtain a temporary work visa, you can get into the United States with that visa and work there only for a certain period of time and for a particular employer and if you enter into America on temporary visas like H visas, O visas, R visas or L visas, you can work and stay in the United States temporarily. However, your prospective US employer will have to file a petition on your behalf with the USCIS and sponsor you for a non-immigrant visa. Moreover, you need to apply for and get the particular type of visa that best suits your need.
Likewise, you can work permanently in America but to do so you must be sponsored by a US employer. If you are hired by a US employer and if that job offer is permanent, you will become eligible for an immigrant visa and with that visa you can get into the United States, obtain a Green Card and work there permanently. United States in certain circumstances also allows exchange visitors and international students to work in the country. In order to work in the country while studying there, an international student must be authorized to work in America and must obtain permission from the Designed School Official. Similarly, exchange visitors must obtain permission from the Responsible Officer, before accepting employment in the country.