Multiple citizenship is simultaneous citizenship in two or more countries (whether it is recognized by all countries or not).
Dual citizenship (being a citizen of two nations) is by far the most common type of multiple citizenship, but nothing in international law prevents individuals from establishing citizenship in more than two countries. Some countries prohibit their citizens from establishing citizenship in another country or only permit it in certain circumstances or for certain countries. Other countries place no restrictions upon citizens wishing to become citizens of other lands.
Each country has different requirements for citizenship, as well as different policies regarding dual citizenship.
For example, a person born in Canada, to a Canadian citizen and an American citizen would have Canadian citizenship by birth, and may also have US citizenship, depending on certain circumstances (the parents' marital status, date of the child's birth, and whether the US citizen parent has met certain physical presence requirements). If the requirements have been met, then the child would also be a US citizen, and therefore would have dual citizenship.
Also, there are some legal issues about dual citizenship and government services. For example, an American citizen with a second nationality and passport may have issues getting a security clearance if they prefer to use the non-American passport or work within the United States government.
While multiple citizenship can be helpful as the individual can carry two or more passports, it is prudent to realize that each citizenship carries responsibilites, such as the risk of conscription or "the draft", as well as pledging allegiance to more than one state, and having to observe travel restrictions, embargoes and sets of laws issued by multiple governemts governing one's behavior domestically and while travelling abroad. Also, a drawback peculiar to a few countries such as the US , citizens are the obligated to pay taxes in both the country of origin and the actual country of residence.
- Dual and Multiple Citizenship (http://www.multiplecitizenship.com/)