Adjustment of Status (AOS) refers to the process by which a foreign-born person who is in the U.S. can change immigration status to that of a lawful permanent resident (LPR, or green card holder) after meeting certain eligibility requirements; and get all the way to approval without having to leave the United States.
This process is, for most types of cases, provided for by § 245(a) of the Immigration and Nationality Act (8 U.S.C. § 1255). It allows applicants to deal entirely with U.S. Citizenship and Immigration Services, rather than traveling to an overseas U.S. consulate.
In fact, the applicant must remain physically present in the United State to apply for AOS. Exiting the country during the application process may be considered as abandonment of the adjustment application, though this is easily prevented by applying for prior permission (advance parole) along with AOS. Some nonimmigrants with temporary work visas are, however, exempt from filing an application for advance parole.
IMPORTANT NOTE: This article does not cover asylees and refugees, who apply for adjustment under a separate section of the law, I.N.A. § 209(b) or 8 U.S.C. § 1159(b). As a result, they need not worry about many of the disqualifications described below, such as unlawful entry to the United States.
Obviously, the first AOS requirement is that the applicant meets the basic eligibility requirements for a green card; and if applying in a "preference category," which is subject to annual limits, that an immigrant visa number is actually available by the time of requesting AOS. (Another way of saying this is that the person's "priority date" has become current.)
Another basic requirement that every applicant must overcome is showing that he or she is not "inadmissible" to the U.S., for example because of a criminal record.
However, there are certain people who, though perhaps otherwise eligible for a green card, are NOT eligible to obtain it by adjusting status within the United States. The main ones are:
Consult an attorney about the various exceptions that might apply to you. Self-petitioners under VAWA are exempt from all of the above.
Someone who meets the various requirements described above and is not subject to any ground of inadmissibility might be able to adjust status to that of U.S. lawful permanent resident. The procedure for doing so is by submitting to USCIS a Form I-485 (Application to Register Permanent Residence or Adjust Status) with all other mandatory forms, relevant documentation, and current processing fees. These must be sent to the USCIS service center for his or her region. (In-person delivery is impossible.)
It is particularly important that you include proof that you were inspected and lawfully admitted to the U.S. with your adjustment packet. Most likely you'd do this by submitting a copy of your Form I-94 Arrival/Departure Record.