Immigration to the United States is regulated by Federal law. The term alien is synonymous with noncitizen. The two basic types of legal aliens are immigrants and nonimmigrants.
Immigrants are persons admitted as permanent residents of the United States.
Nonimmigrants are admitted temporarily as visitors for a specific purpose for example, as tourists, foreign students, diplomats, temporary agricultural workers, exchange visitors, or intracompany business personnel. They are required to leave the country at the end of the time allotted them for this purpose. The conditions for the admission of immigrants are much more stringent, and fewer immigrants than nonimmigrants are admitted. Once admitted, immigrants are subject to few restrictions. They may accept and change employment, and may apply for U.S. citizenship through the naturalization process, generally after 5 years.