Naturalization is the process in which a person applies for and becomes a U.S. citizen. Generally, a person can naturalize after five (5) years of continuously residing in the United States as a Lawful Permanent Resident. Under certain circumstances, a person who received his or her "Green Card" based on marriage to a U.S. Citizen and is still married to and residing with the U.S. Citizen spouse, may apply for U.S. citizenship after three (3) years of continuously residing in the United States as a Lawful Permanent Resident. Certain factors (for example, certain crimes, travel, immigration violations, etc.) may affect a person's ability to become a citizen and applying when ineligible could put a person in a difficult position where they could potentially be stripped of their "Green Card" and removed from the United States.
As part of the process for becoming a U.S. Citizen (with a few exceptions based on age and length of time as a "Green Card" holder), applicants are required to take an English language test and a U.S. Civics/History Exam. Study materials for the exams can be found on U.S. Citizenship and Immigration Services' website.
Benefits to naturalizing include:
Citizenship itself can be a complicated concept. Under the the 14th Amendment of the United States Constitution, any person born in the United States is automatically a U.S. Citizen. Additionally, some people can "acquire" citizenship at birth by virtue of being born to U.S. Citizen parent(s) or can "derive" U.S. Citizen when their parent(s) naturalize. Citizenship through acquisition or derivation can be rather complicated because it involves an examination of immigration laws at the time the child was born or the time the parent(s) became citizens.