Paris has an excellent and extensive Metro network, comprising 16 lines which all run broadly from 5am to shortly after midnight (and a bit later on Friday and Saturday nights). It’s one of the oldest systems in the world, and operates 300 stations so wherever you want to go in the city chances are it’s well connected by Metro.
Single journeys cost €1.80, while books of ten tickets are available at a discount of €14.10. Day passes (known as Ticket Mobilis) are available for €7, and cover unlimited journeys within the two central zones of Paris’ underground system.
For more, visit the RATP website - http://www.ratp.fr/en/
The Metro is not to be confused with the RER, the regional commuter service which serves a number of the main stations in the city centre. If you arrived at Charles de Gaulle, you likely caught the RER to the city centre. Metro tickets are interchangeable with RER services within the city centre, but your €1.80 Metro single doesn’t cover you for journeys outside the city centre.
The excellent www.parisbytrain.com offers full information on all of Paris’ public transport ticketing quirks and various options for passes.