The basic Métro ticket is a small piece of cardboard (formerly mauve, now white) with a magnetic strip that costs €1,80. It's issued by RATP, the transportation authority for the Paris region.
You can save money by purchasing a carnet of 10 tickets for €14,10 from vending machines near the turnstiles in Métro stations. (A carnet is simply a stack of individual tickets.) If you're traveling with a children, see the "Tips" section below.