Immigration law governs all of the rights and responsibilities of people from one country living or visiting the US. Specifically, immigration law determines the individual's rights to live, work, or visit the US if he or she is not a resident.
There are many ways of legally being in the country, including being a citizen, becoming a permanent resident (also known as having a green card), or possessing a valid visa which allows for travel or work. The US government must approve these different statuses, with a US citizenship application, green card application, or green card renewal all determined by federal agencies. Without proper approval or documentation, it is illegal to live or work in the country, and certain criteria, such as criminal records, may also be obstacles to immigration.
Immigration law is extremely complex, with many different government agencies overseeing and enforcing immigration law. These agencies have different interests and regulations concerning immigration law. For example, whereas the Department of Homeland Security focuses on issues such as border patrol, the Department of Labor makes decisions about immigrants working in the US.
Understanding immigration law is important if you are not a citizen but are visiting, working, or living in the US.