A stylus is a tool with a hard, pointed end that you use for writing or drawing. Today, you can also a stylus to virtually "write" on an electronic tablet, computer, or smartphone.
Styli (or styluses) were originally used in ancient Greece and Rome to scratch writing into wax tablets; the blunt end of the stylus worked as an eraser, smudging the words out. Artists also use styli for shaping clay. If you're lucky enough to have an old-fashioned record player, you know that stylus also refers to the "needle" that fits into the grooves of the record and plays it.