In addition to the 14 lines of the Paris subway, the Métro (from Métropolitain), several lines of RER commuter transit trains cross the city and serve the suburbs. They allow you to reach Versailles and its royal palace, Disneyland, the Stade de France, and Roissy-Charles de Gaulle airport.
Paris has several recent tram lines running along the city limits or serving the outskirts, as well as many bus routes crisscrossing the city. Despite a growing number of bus lanes, buses still get stuck in traffic a lot and are not a good means of transport to get from point A to point B – unless you are there for the ride, and not the destination.

The easiest way to get from point A to a point B that is too far to walk (I assume you are here to see things) is the metro. Nowhere in Paris are you more than 500m from one of the over 300 metro stations.

The only stations not connected to the métro/RER network are the bottom and top stations of the Montmartre Funicular.