Naturalization refers to the process for obtaining US citizenship. Anyone born in the United States is automatically a US citizen (except children of foreign diplomats who become permanent residents), and some children born abroad may be citizens by virtue of "derivation" from a US citizen parent or parents.
Note that USCIS has issued excellent information about U.S. citizenship, which is available here.
In general, if one of the parents is a US citizen who has lived in the United States for a certain period of time after the age of 14, the child will be a US citizen. The rules for children born abroad of US citizen parents are complicated, and have changed several times over the years. Therefore, anyone who has a US citizen parent should check with an immigration attorney to be see whether that person has derived US citizenship at birth. Some people spend years jumping through CIS hoops to obtain a series of temporary visas without realizing that they are US citizens!
Some aliens may also "acquire" US citizenship by virtue of the naturalization of his or her parent(s). There is an expedited process for naturalization of children under the age of 18 and for adopted children of US citizens.
Proof of US citizenship can be a US birth certificate, consular report of birth abroad, US passport, Naturalization Certificate or Certificate of Citizenship. The Certificate of Citizenship is a document available to those who have derived or acquired US citizenship, but were not born in the United States. Before filing the N-600 to obtain a Certificate of Citizenship, which may take a year or more, be sure to check whether the person is eligible directly for a US passport, which can take only a few months.
We advise anyone with a Naturalization Certificate or Certificate of Citizenship to apply also for a US passport as additional proof of identity. If the Certificate is lost, it could take a very long time to replace it, and without a US passport, there would be no proof of citizenship.
In most cases, a person who has been a lawful permanent resident (LPR) for at least 5 years is eligible for naturalization.
The period of required lawful permanent residence is reduced to 3 years for the spouse of a US citizen, but the spouse must show that he or she is still married. The requirements are also eased for children and for those who have served in the US armed forces.
Please note that some children under 18 years of age may automatically become US citizens when their parents naturalize. This may depend on the particular naturalization process of the parent(s).
Note that there may be waivers of these factors. If any of the following apply to your situation, you should consult with an attorney before filing for naturalization. This is important because the CIS can re-open your green card case if new information is uncovered at the naturalization interview.
There are many benefits of applying for US citizenship. Currently, US citizens can vote, run for most public offices, apply for all types of financial aid for education, apply for all government grants for research as a principal investigator, and receive all public benefits. US citizens are also not subject to the many grounds of deportability that can affect permanent residents, such as committing a crime.
One reason that many LPRs give for not applying for US citizenship is that they do not want to lose citizenship from their home country. However, not all countries revoke citizenship when one of their nationals obtains US citizenship. Germany, for example, does not allow dual citizenship, while Canada and France do. Check with an immigration attorney or the appropriate embassy or consulate to see whether dual citizenship is permitted (or at least tolerated).
Unfortunately, it is impossible to guess what Congress will do in the future. The best way to protect against a tide of anti-immigrant sentiment that emerges periodically (for example, the Republican Congress of 1996) is to become a US citizen.