Originally the singular second person pronoun in English; at that time "you" was always plural. Eventually "you" became formal and "thou" became informal, and to be polite people generally used the formal form. The Friends (Quakers) felt that no one deserved to be elevated above others, so they always used the informal form and preserved it.
Gorgonzola notes: "I would hope that when using 'thou', people also use the appropriate verb conjugation (thou art, thou wert, thou hast, etc)."