The options below include transportation on métro lines, city buses, tramways, and the Montmartre funicular.  The most common choices for visitors are:

Single Ride Ticket:  Called a “T+” ticket.  Cost:  €1.70 for single tickets.  Purchase a pack of ten, termed a “carnet” for €13.30.  Transfers between métro/RER lines are permitted if the rider does not exit the system.  No transfers between bus and métro.

Paris Visite Pass:  A multi-day travel and discount pass heavily marketed to tourists.  It’s not a good value.  Don’t buy it.

Mobilis:  Essentially, it is an all-day unlimited ride T+ ticket.  A one-day pass is €6.60 for zones 1-2 (compared to the pricey Paris Visite at €10.55 for zones 1-3).  

Navigo Découverte:  Valid only Monday through Sunday but probably the best value if moving around the city 4 or more days within this time frame.  It involves purchasing a €5 plastic card holder from a ticket agent.   A driver license size photo needs to be attached.  Charge it up at a métro station kiosk for €19.80 and ride to your heart’s content in Paris center for a week. Métromaniacs can ride for a month for €65.10.

I get the best value and most use from unlimited ride passes.   I tend to use public transportation more if I don’t need to ration individual tickets.  

The easy way to purchase T+ and Mobilis tickets and charge Navigo Découvertes is to use the multi-lingual automated kiosk at nearly every métro station.  Otherwise, deal with a ticket agent speaking limited English through inch-thick bullet-proof ticket-window glass.

When using a ticket to travel, hold onto it until exiting the bus, métro, or train station.  Although it is uncommon, inspectors can assess a fine on the spot to those not holding a ticket.

BTW, a ticket is called a “billet” in French.

For métro/bus/RER maps, go to: www.ratp.fr