Paris, the most visited city in the world with 45 million tourists a year and one of the easiest to navigate public transportation.
There are 14 subway lines + RER (regional express network) + buses.
If you kept green or yellow tickets from an earlier trip, they are still valid. Metro tickets are good for 2 hours as you do not go out of the network and whether you should continue by bus, you will need another ticket.
The Noctilien: If the subway is already closed because you missed the last one, do not worry, there is this bus for those who like to walk in the Parisian night; from 0:30 to 5:30.
The Balabus: special line that gives you the opportunity to know the city from east to west. From Gare de Lyon to the Grande Arche de la Defense. Runs from April to September only. The cost is 1 to 3 metro tickets depending on the distance traveled. You will pass the Bastille, the island St. Louis, Notre Dame, the Louvre, Les Invalides, the Eiffel Tower, Champs Elysees, etc.
The RER rail network that covers a large portion of the island-of-France. If your RER station is connected to the metro, the same ticket will be valid.