A U.S. citizen can petition for legal permanent resident status for his/her spouse who is a foreign national. Spouse of a U.S. citizen is an “immediate relative”. Once the bona fides of the marriage are established, an immigrant visa is immediately available for a foreign national spouse.
Spouses of legal permanent residents can immigrate under the second preference category along with their dependents. They have to wait until immigrant visas are available.
The foreign national seeking legal permanent resident status has to pass the interview, medical examination, and background check. The petitioning spouse has to file an affidavit that (s)he will support his/her spouse until (s)he becomes a U.S. citizen or can be credited with 40 qualifying quarters of work in the U.S.