A citizen is a person who resides in a country "legally" (with the correct documentations). There are two types of citizens: native-born and naturalized. In America, native-born citizens are persons who were born in the U.S., automatically making them citizens. Naturalized citizens were not born in the U.S., so they had to complete a legal process to gain their citizenship.
Depending on the laws that were in effect when a person was born, one may already be a U.S. citizen, despite the fact that he/she was born in a different country. A child can be granted citizenship if his/her parents are U.S. citizens. In some cases, a child can also be granted citizenship if his/her parents are not U.S. citizens, but complete the naturalization process before the child turns 18 (Martin). Children must be permanent residents and under legal custody of the naturalized parent or parents (Washington New Americans).