A citizen is defined as an individual who owes allegiance to a government and has certain rights and duties under that government. There are two main ways to become a US citizen - by birth within US territory or by going through the naturalization process. Citizens have both civic duties like obeying laws and paying taxes, as well as civic responsibilities fulfilled voluntarily like voting, community service, and staying informed on issues. Being a good citizen involves traits like trustworthiness, respect for others, responsibility, and participation in one's community and elections.