Adjustment of Status describes one manner in which certain foreign nationals who are already present in the United States may apply for a green card. If you are eligible for Adjustment of Status, you will not have to leave the U.S. to obtain your green card. By contrast, anyone not eligible for adjustment of status must apply for a green card at a foreign consulate outside the United States via Consular Processing.
Typically, in order to be eligible for adjustment of status, you must demonstrate that you: (1) entered the U.S. legally, and (2) never violated immigration laws. However, there are certain exceptions to this rule such as in the case of immediate relatives and those eligible under Section 245(i).