What Is Naturalization?
Naturalization is the application procedure that is utilized by aliens that want to become US natural citizens. If someone is born in this country, then under the Constitution they are automatically a U.S. citizen, regardless of whether or not their parents were here legally or illegally.
If someone is not born in this country and they have a green card, then under certain conditions they can later apply to become a U.S. citizen and that procedure is called naturalization. It is also possible if you receive your green card through employment or through other means. After 5 years then you can apply to become a U.S. citizen.