What is deportation? It's commonly understood to mean when someone is sent out of the U.S. by the immigration authorities. The preferred technical term these days, however, is "removal."
Whatever word you use, there are many circumstances under which the immigration authorities can enforce someone's departure from the U.S., ranging from officers catching immigrants attempting to cross the U.S. border and immediately returning them, to removals of aliens in detention, to issuing an order of removal to an immigrant whose immigration court case has been denied (perhaps after several appeals).
The agency on the frontlines of immigration enforcement matters is Immigration and Customs Enforcement, or ICE. Everyday immigration matters, such as application processing -- which can sometimes aliens into removal proceedings, when the applications are unsuccessful -- are handled by U.S. Citizenship and Immigration Services, or USCIS. And immigration court proceedings, at which non-citizens may defend themselves against removal and establish a right to remain in the U.S., are handled by the Executive Office for Immigration Review (EOIR).
When most people think of an immigrant getting deported, they think of undocumented immigrants, often called illegal aliens. This refers mainly to people who either crossed the U.S. border illegally, as well as those who stayed beyond the permitted time on a temporary (nonimmigrant) visa. Indeed, many undocumented aliens enter and are removed from the U.S. each year. But they are not the only aliens who can be deported.
Any immigrant who violates the terms of his or her stay can be deported or removed from the United States. This might include a tourist who accepts a job (not allowed), or a student who fails to pursue a full-time course of study.
In addition, any immigrant who hasn't yet become a U.S. citizen, including a green-card holder (permanent resident) can be deported if he or she does something that's on the immigration law's list of grounds of deportability.
The grounds of deportability are found in Section 237 of the Immigration and Nationality Act (I.N.A.). For example, non-citizens can become deportable if they:
This is just a quick overview of these grounds -- do not rely on this list to analyze your case, but look at the law itself, linked to above, and consult with an experienced U.S. immigration attorney. Also note that despite the seriousness of being accused of being deportable, some grounds can be overcome by requesting a waiver.
Non-citizens have the right to a lawyer, as well as other rights under the U.S. Constitution. The immigration authorities cannot simply deport someone without providing a chance to be heard.
Of course, the authorities often try to make the process go quickly, by asking the immigrant to sign something agreeing to depart without a hearing. In some cases, when the immigrant really is in the U.S. illegally with no defense to removal, leaving voluntarily can be the best way to go, because it avoids having an order of deportation on one's record.
But anyone who believes they might have a right to remain in the U.S. should insist on their right to a lawyer (which they will, unfortunately, have to pay for on their own) and to a hearing on the merits of their case.
A non-citizen who hasn't yet entered the U.S. does not have the same rights, and is subject to expedited removal. In other words, the person can simply be refused entry unless he or she has a legitimate fear of persecution if forced to return to his or her home country. In the latter case, the immigration officials must allow the person's asylum claim to be heard.
Immigration court proceedings are administrative, meaning they're less formal than ordinary court proceedings, and the usual rules of evidence don't apply. USCIS will be represented by an attorney. The judge as well as both attorneys may ask questions of the immigrant, and either attorney may bring in witnesses to testify.
The hearing will last as long as is needed to present and hear all the evidence. The judge may issue a decision at the end of the hearing, or later. If it's a negative decision, the judget will issue an order of removal, which becomes final as soon as the allotted time for appeal is over. Negative decisions can be appealed to the Board of Immigration Appeals (BIA) and then through the federal court system.