Citizenship is the relationship between an individual and a nation-state whereby the state grants the citizen certain rights, such as the right to vote, work, and own property, and in return, the citizen accepts the responsibility of upholding the laws and customs of that state. Citizenship unites different people under a common identity.

Traditional means of acquiring citizenship and the corresponding passports have been birth, naturalization, and marriage. Naturalization is the process by which a resident of a country can acquire citizenship usually by residing in that country for a certain number of years.

Since 1984 investment in the host country has been another way of acquiring citizenship.

source