Kim Dotcom and three others were arrested in New Zealand after a request for "mutual legal assistance" by the United States Government - which plans to extradite the accused.
The men have not been charged by police in New Zealand and are being held in custody on the warrant issued by the US Government.
Extradition is the official process allowing for the surrender of a suspected or convicted criminal from one country to another.
Requests are made under the Extradition Act 1999, any relevant treaty and the law of the foreign country. New Zealand signed an extradition treaty with the US in 1970.
Normally, a formal request for extradition is received through diplomatic channels. The Minister of Justice may request an arrest warrant and then the District Court determines whether the individual is eligible for surrender.
If so, the matter is referred to the minister, who makes the final decision.
A country can also request the arrest of an individual before making a formal request for extradition.
This is done when it is necessary to issue an arrest warrant urgently - often because of flight risk or the seriousness of the offence.
A request for extradition is then made and the court process begins.