The vocative case is the case used for a noun identifying the person being addressed, found in Latin among other languages. In Latin the vocative case of a noun is the same as the nominative, except for masculine singular second declension nouns. An example would be the famous line from Shakespeare, "Et tu, Brute?" (And you, Brutus?, commonly translated as You too, Brutus?), where "Brute" is the vocative case, whilst "Brutus" would be the nominative case. When Latin names are translated into English, the nominative case is usually used.
In English the vocative case is not marked, but English syntax performs a similar function; witness: "John, could you come here?" or "I don't think so, John", where "John" is neither subject nor object of the verb, but rather indicates the person to whom the statement is being addressed.