Citizens of foreign countries who wish to obtain permanent resident status in the U.S. generally must obtain immigrant visas at the nearest Consulate or U.S. Embassy.
In consular processing, the interview for an immigrant visa takes place at a U.S. embassy or consulate located (with some exceptions) in the foreign person's country of nationality.
When your family-based immigration or employment-based immigration petition is approved, USCIS sends it to the National Visa Center (NVC). After NVC receives the petition, it will create a case record and assign a case number, usually within 24 hours of being received from the USCIS. NVC retains the case until it is ready for adjudication by a consular officer abroad. Petitions may remain at NVC for several months or for many years depending on the visa category and country of birth of the visa applicant. When an applicant's case is about to become current (a visa is likely to be available within the year), the petition is forwarded to the appropriate U.S. embassy or consulate overseas.
Adjustment of status ("AOS") is a procedure that allows an eligible applicant to become a lawful permanent resident of the United States without having to go abroad and apply for an immigrant visa. Another alternative to AOS is Consular Processing.
Adjustment of status concerns your personal history, such as health, finances, places of residence, family, and political and criminal background.
Between the time the adjustment of status's application is filed and it is approved, the applicant is considered to be in legal status as an "applicant to adjust status."
If a person entered the U.S. illegally, that person is barred from adjusting his status to permanent resident even if he or she marries a U.S. citizen or otherwise becomes an immediate relative. An immediate relative must meet the eligibility requirement of being admitted or paroled into the United States.
Generally, person must be in status all the times in order to file Adjustment of Status. Exceptions to this are, if the person had stayed illegally for less than 180 days, he/she can apply for AOS. If he/she had stayed illegally for more than 180 days but the priority date is earlier than January 14, 1998 or qualifies under LIFE act, he/she can still apply for AOS by paying $1000 fine. All other persons must do consular processing abroad.