Duden lists four meanings for Diener. The first is a man who works for pay in a private household. Duden’s example is ein livrierter Diener bediente die Gäste, a servant in livery served the guests. A stummer Diener, a silent servant, is a serving table.
The second and third meanings are marked as elevated language. A Diener can hold a public office, as in ein Diener der Kirche, des Staates, a servant of the church, of the state. He can also place himself freely under a cause and work for it, as in ein Diener der Wissenschaft, der Kunst, a servant of science, of art.
The fourth meaning is a bow, especially by boys. Einen Diener machen means to bow when greeting someone. Duden marks this meaning as veraltend, going out of use. It traces the bow to the old polite formula gehorsamster Diener!, most obedient servant.