The U.S. immigration system is based on three general concepts or stages:
- Being given a label,
- Being admitted, and
- Being granted status.
“The Label” is the process by which a non-U.S. citizen is identified as a specific kind of “category” or person. The categories are defined by immigration law (which is set out in the U.S. Immigration and Nationality Act, and implemented by Title 8 of the Code of Federal Regulations). The most common categories (or labels) is “Immigrant” and “Non-immigrant”.
An Immigrant is a person who intends to live in the U.S. indefinitely. These persons also are called permanent residents, or greencard holders (because their ability to live indefinitely in the United States is demonstrated by a greencard). Examples of Immigrants are: Spouses of US Citizens, Outstanding Researchers, and Multinational Managers.
A Non-immigrant is a person who is permitted to be physically in the United States only for a specific, temporary period of time. Often the end-date of the visit is set at the moment of arrival. In other cases, the end-date might be more flexible. But in all cases, an end to the visit is expected to occur at some point. There are many, many different types of non-immigrant, and they each are defined by the regulations mentioned above. Examples of non-immigrants include: H-1B Specialty Occupation Workers, F-1 Students, and K-1 Fiancées.
Sometimes the foreign national is the person who asks for the label, but in other cases it might be a U.S. person or entity (which can include a lawful permanent resident or greencard-holder who already is in the U.S.). Sometimes label must be approved before any further steps can be taken, but in other cases, a foreign national may request a label at the same they ask to be admitted and/or ask for a grant of status. The usual way of requesting a label (category) is by filing a petition, and the person who asks for a label usually is called a petitioner.
When a foreign national physically arrives in the United States, they usually speak to a customs officer either at the airport (before or after the flight) or at the land crossing. Surprise! The customs office also is an immigration officer. They are part of the U.S. Bureau of Customs and Border Protection (CBP). The short conversation you have with them is called an “inspection”. Once you have been “inspected” by the officer, your passport (usually) is stamped by the officer and you are permitted to walk (or drive) away from the desk and into the wilds of the United States. This is called “Admission”. These are very important concepts. The law (generally) requires that a person be inspected and admitted in order to be “legal” in the United States.  , 
To be admitted, you must first identify yourself using the label that (probably) was already assigned to you (see Part 1 above). This begins with identifying yourself as an Immigrant or Non-immigrant – but more than this; you have to identify the specific kind of immigrant or non-immigrant that you are. For example, are you the “Spouse of a US Citizen” or are you an “H-1B Specialty Occupation Worker”? For most people, this is done by presenting a visa stamp.
During the “inspection” (the short conversation with the customs/immigration officer), you must also describe a situation that fits precisely within the rules of the specific kind of Non-immigrant or Immigrant you want to be. If you want to enter the United States to be an “F-1 Student” (for example), you should be able to show that you will go to school in the United States. If you want to enter as a “Fiancée of a US Citizen” you should be able to talk about how you will get married.
The Grant of Status
“Status” is the bundle of legal rights and obligations a non-U.S. person has while in the United States. Status can be granted by the following agencies: the CBP, the U.S. Citizenship and Immigration Service (USCIS, formerly known as the Immigration and Naturalization Service or INS), or by an immigration judge. They grant status only after they confirm that you are 1) eligible for the category (the label you want) and 2) eligible for admission. Often, though, the process of being approved for a category, approved for admission, and granted status, pass so quickly in sequence that the foreign national involved does not know that three separate determinations occurred.
Each label (immigrant or non-immigrant category) available to a foreign national is based on the activities that will be undertaken while in the United States. This is why, during inspection and admission, a foreign national must describe a situation that fits within the activities permitted to specific category of immigrant or non-immigrant. Once a person is granted the status that goes along with that label, they have the right to engage in the activity. For example, once a person is awarded permanent resident status, they have the right to live indefinitely in the United States. Once a person is awarded F-1 student status, they have the right to attend school.
But the permission to engage in an activity is really an obligation to engage in that activity. When a person enters the United States saying they want to do one thing, but then never doing that, they risk losing the status given to them. It is considered a “Violation of Status” to not do something you said you would do. If a person is admitted to the United States as a “Fiancée of a US Citizen” but then never marries that person, s/he has violated terms of the status.
One of the “terms” of admission is to leave by a certain date. The period of time you are allowed to be in the United States is called your “stay”. If you remain in the U.S. past the end date of your “authorized stay”, then you have “overstayed”. If you overstay, you become “unlawfully present”. If a person is unlawfully present for more than 6 months, they can become subject to a rule that says they must remain outside the United States for three years before being eligible to be admitted again. If a person is unlawfully present for more than one year, the bar to re-admission is ten years. In some cases, waivers of these bars to re-admission are available.
 Categories also are called classifications.
 Thus, for example, the infamous “H-1B” visa classification gets its name from Title 8 of the Code of Federal Regulations, Section 101 (Definitions), subsection 15 (Non-immigrants), and sub-subsection (H)(1)(B).
 At one point, long in the distant past, the U.S. immigration system was more uniform, but this has been lost over decades of piecemeal amendments and exceptions. There now are bewildering differences in how a person may enter the U.S., depending on where they begin the trip, the activities they plan for the United States, and other factors.
 The exception to this is Canadians who have Nexus cards. They do not stop to chat with customs/immigration officers during their drive across land borders. The rules of the Nexus card and the arrangement between Canada and the United States means that they still are considered to have been inspected and admitted.
 Let’s return to the concept of labels for a moment. Because the U.S. Immigration system is pretty close to completely broken, amendments and tweaks to immigration law that have occurred over the decades have resulted in the formation of a less rigidly defined third group – the “people who are not illegal.” This group includes people who might have done something not normally permitted by the terms of their admission, but whom the law says are “ok” anyway. We will talk about this concept a little later.
 Subject to certain, tightly defined exceptions (such as for Canadians), a failure to be inspected and admitted probably means you are “illegal”.
 There are certain exceptions to this for Canadians and for people entering the United States under the Visa Waiver program.
 If the facts you present do not fit within the requirements for the category (label) you want, the officer usually will refuse to admit you to the United States. Sometimes they will simply refuse admission. Sometimes they will direct you to obtain a different visa (from a U.S. Consulate) and they try to seek admission again. If a person lies during inspection and the officer discovers that, and the truth is VERY different from the category of admission you wanted, the officer might decide that you were lying to obtain admission. This can be characterized either as misrepresentation or fraud, and in the worst cases, it can result in a five-year bar against being able to seek admission to the United States again.
 Unlawful presence is not the same as being illegal. The differences are beyond the scope of this post.