What is a Certificate of Citizenship?

From the United States Immigration Support:

The Certificate of US Citizenship (form N-560 or N-561) is a document issued by the United States government as proof of U.S. citizenship. Individuals who are eligible to apply for the United States Certificate of Citizenship include those who obtained U.S. citizenship while residing in the United States or individuals who were born outside the United States to U.S. citizens. Specific details of eligibility include:

■ Your parents are U.S. citizens, but you were born abroad.
■ At least one of your parents became naturalized when you were under 18 years of age and met the special conditions of the law.