Metro is one of the most convenient ways to move around Paris. There are some 300 stations, 16 lines, it is easy to navigate, even if you don't speak French. The lines are colored and numbered.
There are multiple ticket options, and it is very easy to get confused. For the full list of options and fares, please visit the official Paris metro website, RATP.
If you are visiting Paris as a tourist, here is a short summary that might be of use:
one-way ticket is called "Le ticket t+". It is €1.90.
a book of 10 t+ tickets is called "carnet", €14.90 for adults and €7.45 for children 3-10 years old (children under 3 years old travel free) - makes sense to buy a carnet if you plan to often use metro in Paris.
unlimited daily ticket, or Mobilis - use it to travel within Paris. There are some restrictions though. Learn more at RATP website. €7.50.
Paris Visite is one of the most cost efficient options to consider. It is designed for visitors, and is available for 1, 2, 3 or 5 consecutive days for zones 1-3 or 1-5. Starts from €12.00. Learn more at RATP website.
Please do not discard your ticket until you leave the metro station - there are random checks at the stations, and if you cannot show your ticket, you will be a subject to a fine.
You can buy tickets at metro stations. If you pay by credit card, make sure it has a chip, otherwise your card will not be accepted. It is always best to have cash, both bills and coins, should the credit card payment be declined for any reason.
View interactive Paris metro map - includes zones in Paris metro
Metro hours in Paris are 6am to 0.30am.