The Knights Hospitaller were a group of military knights fighting in Jerusalem. They are the group that became the Knights of the Order of St. John of Jerusalem and, eventually, St. John Ambulance.
The Hospitallers are a Catholic military order founded at Jerusalem in the 11th century. They later became the Sovereign Military Order of Malta. Today their headquarters are in Rome. The order continues its humanitarian tasks in most parts of the modern world under several slightly different names.