From the Wikipedia
The Knights Hospitaller (also known as the Sovereign Military Hospitaller Order of St. John of Jerusalem of Rhodes and of Malta, Order of St. John, Sovereign Order of Saint John of Jerusalem of Rhodes and of Malta, Knights of Malta, Knights of Rhodes, and Chevaliers of Malta; French: Ordre des Hospitaliers) is a Christian organization that began as an Amalfitan hospital founded in Jerusalem in 1080 to provide care for poor and sick pilgrims to the Holy Land. After the Christian conquest of Jerusalem in 1099 during the First Crusade it became a religious/military order under its own charter, and was charged with the care and defense of the Holy Land. Following the loss of the Holy Land by Christian forces, the Order operated from Rhodes, over which it was sovereign, and later from Malta where it administered a vassal state under the Spanish viceroy of Sicily.
When Napoleon captured Malta in 1798 the Knights ceased to be associated with any one place, but gave rise to successors in existence until the present.
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In 1834, the revived Order established a new headquarters in Rome. The Sovereign Military Hospitaller Order of Saint John of Jerusalem, of Rhodes and of Malta, better known as the Sovereign Military Order of Malta (SMOM), remains a Roman Catholic religious order which along with another order is a virtual Sovereign State with membership of many international bodies and observer status at others (such as the United Nations). The Grand Masters of the two Orders are thus Papal Viceroys who provide Vatican diplomacy with procedural support for making motions, proposing amendments and requiring votes in the sphere of international diplomacy. Both Orders maintain Embassies in and receive Ambassadors from the (mostly majority Catholic) countries who recognise their Sovereignty. Their claims of sovereignty are disputed by some scholars and remain unrecognised by many other countries.
http://en.wikipedia.org/wiki/Knights_of_maltahttp://en.wikipedia.org/wiki/Sovereign_Military_Order_of_MaltaUpdated with added information