Believing themselves superior, Romans carefully parceled out privileges to deserving foreigners during the Republic and the Empire prior to the granting of universal citizenship (the Constitutio Antoniniana under Caracalla). Some of the terms are debated -- for instance, was it possible to receive Roman citizenship and privileges without obligations? Some international relationship terms come from personal relationship terms. Here are some of the terms you may come upon when reading about the expanding Roman Empire.
You may also see some of these terms in Latin love poetry.