Immigration law is the name given to the branch of law that covers U.S. citizenship, loss of citizenship, and the admission and removal of aliens. Legal immigration involves immigrants, i.e., persons seeking a permanent residence card or "green card," and non-immigrants, persons seeking temporary entry to the U.S. under a non-immigrant visa category. Immigration also encompasses asylum, naturalization, denaturalization, deportation, as well as immigration crimes, including illegal immigration, or aliens who enter the U.S. and obtain work illegally.