Dual citizenship is when two separate nations consider an individual to be a citizen of their country. It is also possible to be a citizen of three or more countries. There are no specific forms or applications required to have dual citizenship and there is no certificate that accompanies it.
Dual citizenship happens when a person applies and is allowed to be a citizen of a nation that is not their place of origin. On some occasions, dual citizenship happens automatically. For example, if an individual was born on foreign soil to parents that are citizens of The United States of America.