If you are applying for a green card (U.S. lawful permanent residence), a nonimmigrant (temporary) visa, or for certain other immigration benefits, you will need to deal with some combination of U.S. government agencies. The primary possibilities are as follows:
Its website is www.travel.state.gov. The DOS acts through U.S. embassies and consulates located around the world. If you’re coming from outside the U.S., you’ll be dealing primarily with a U.S. consulate. Even if you’re currently in the U.S., you may eventually have to travel to a consulate to complete your application. Not all U.S. consulates provide visa-processing services.
To find out more about the policies and procedures of the U.S. consulate nearest your home, go to www.usembassy.gov. Note that you cannot normally apply for an immigrant visa (the kind that lead to permanent residence or a green card) in a U.S. embassy or consulate outside your home country, unless the U.S. has no diplomatic relationship with the government of your home country. You can apply for nonimmigrant visas (such as tourist visas) in third countries, so long as you have never overstayed your permitted time in the United States, even by one day.
This is a private company under contract to the DOS. It's role is to handle case files during certain intermediate parts of the green card application process. After USCIS approves a visa petition by a U.S.-based family member or company, the NVC is given the file and handles the case until the person's priority date has become current and a visa is available, at which time the NVC will forward the file to the appropriate U.S. consulate or USCIS district office.
Formerly Immigration and Naturalization Services (INS), its website is www.uscis.gov. It is an agency of the Department of Homeland Security (DHS). Even if you’re living outside the United States, you or your petitioner (the family member or employer sponsoring you) may have to deal with USCIS, particularly if you’re applying for a green card rather than a temporary visa.
Most green card applications must be started by a U.S.-based family member or company filing a visa petition with USCIS. USCIS has various types of offices that handle immigration applications, including service centers and lockboxes (large processing facilities that serve a wide region, which you cannot visit in person), district offices (which you can visit in person, to pick up forms, request information, and attend your green card interview), sub-offices (like district offices, but smaller and with more limited services), Application Support Centers (where you go to have fingerprints taken and in some cases pick up forms or turn in applications), and asylum offices (where interviews on applications for political asylum are held).
Its website is www.cbp.gov. It is also under DHS, responsible for patrolling the U.S. borders. CBP’s role includes meeting you at an airport or other U.S. entry point when you arrive with your visa. At that time, it will do a last check to make sure that your visa paperwork is in order and that you didn’t obtain the visa through fraud or by providing false information.
Its website is www.dol.gov. The DOL acts primarily through its Employment and Training Administration, at www.doleta.gov. If your visa or green card application is based on a job with a U.S. employer, certain parts of the paperwork may have to be filed with and ruled on by the DOL. The DOL’s goal is to make sure that by hiring immigrant workers, U.S. employers are not making it harder for U.S. workers to get a job, and that you are being paid a fair wage (and one that doesn’t act to lower the wages of U.S. workers).
Although you need not learn a great deal about these various immigration-related agencies, it’s especially important to keep track of which of them has your application at any given time. Sending correspondence to the wrong agency can delay your application, and in order to check on your application's status using USCIS's online case status service, you'll often need to know which often it's held at.
It is doubly important to know where your file is being held if you ever change your address, because you will need to advise the office that actually has control of your application. That’s true even if you’ve separately advised USCIS using the Change of Address form (AR-11). USCIS offices don’t communicate well with each other, and if you tell one of them about your change of address, it may not pass the word on to the one that actually has your file. The result can be that you don’t receive notification of important requirements or interviews.
Read Make Sure Your Immigration Application Doesn't Get Lost in the Process for tips on staying on top of things.