Styli are used either for ink work, or used to engrave into metal or clay.
"Stylus" is also the term usually used by those in the sound recording industry for what is more commonly called a phonograph or gramophone "needle", used to play back sound on analogue disc records, as well as to record the sound indentations on the master record.
Today, the term "stylus" often refers to an input method usually used in PDAss. In this method, a stylus that secretes no ink touches a touch screen instead of a finger to avoid getting the natural oil from one's hands on the screen.