There are a number of basis on which a person may apply for a green card (permanent residency). It can be a spousal relationship with US citizen or permanent resident. US citizens can petition for foreign-born spouses as immediate relatives. Lawful permanent residents can petition for their spouses, however, unlike with US citizen spouses, the petition falls into the second preference family category.
Also parents, unmarried child under age 21, the unmarried son or daughter over age 21, the married son or daughter, or the brother or sister of a US citizen may apply for adjustment of status to permanent residency when a visa petition is approved on their behalf. Unmarried son or daughter of any age of a lawful permanent resident with an approved family-based visa petition may also apply.
Foreign nationals for whom an employer filed an immigrant petition and got an approval may apply for adjustment of status to permanent residency. One may apply for adjustment of status only when an immigrant visa number is available from the State Department unless s/he is in a category that is exempt from numerical limitations. Immediate relatives of United States citizens are exempt from this requirement. Immediate relatives of U.S. citizens are parents, spouses, and unmarried children under 21.
For family members of lawful permanent residents, law limits visa numbers every year. This means that even if the INS approves an immigrant visa petition for you, you may not get an immigrant visa number immediately. In some cases, several years could pass between the time INS approves your immigrant visa petition and the State Department gives you an immigrant visa number.
Asylees or refugees may apply for adjustment of status if they have been in the United States for at least a year after being given asylum or refugee status and still qualify for asylum or refugee status.
Cuban citizens or natives who have been in the U.S. for at least a year after being inspected, admitted, or paroled into the United States may also apply for adjustment of status. Their spouses and children who are residing in the U.S. may also be eligible for adjustment of status.
Continuous residents of the U.S. since before January 1, 1972 may be eligible to apply for adjustment of status.
There may be other basis for adjustment of status.