By far the most famous Notre Dame is located on the Ile de la Cité, in Paris, France. Actually, however, the gothic Notre Dame cathedral is not a uniquely Parisian affair.
Notre Dame literally means "Our Lady", refering to Mary, the mother of Jesus. While the Paris tourism bureau would probably like you to think that they've got the best one, there are really beautiful Notre Dame cathedrals scattered all over France... Our Lady of Rouen, Our Lady of Chartes, Our Lady of Toulouse, Our Lady of Amiens, etc.