These passes will usually get you into many of the Paris museums or attractions.
They are available for different lengths of time like 1, 3, 5 or even 7 days. Sometimes they are actually offered by the city or the city's museums or the city's transit authority. sightseeing city passes
Sometimes they are offered by a private company that has an agreement with the city or the attractions that are included.
The Carte Muses et Monuments or Paris Pass in Paris gets you into 63 museums and monuments in Paris and the surrounding area. Yes that's a lot of sightseeing to do. You get an unlimited number of visits and get to go in a special entrance so you can skip the lines. This is a great deal if you're into museums... you can pop in and out of the Louvre and others and have time in between for a head clearing walk in the park.
Whenever you consider city passes or transit cards or museum cards... just consider if they really save you money. We've evaluated others and chosen not to get them. If you're only seeing one museum or taking one metro ride, just pay for the one, the "pass" will cost you too much.
So when you're planning a trip to visit Paris with it's many museums and attractions, check into a city pass. You can find out about these on the internet. You can buy these ahead online too.
You can almost always wait and buy them at one of the museums included or at a tourist information office. One way or the other, it's good idea to have the information ahead and have time to think about it so you don't have to waste time deciding when you are standing in front of the museum. These passes will be validated at the first attraction you visit, and your time will be calculated from that.
Look at the price of the Paris Pass and which museums and attractions are included. Are you even interested in museums? Do you want to go to many of the attractions included, or are there many that just don't interest you?
Will you have time to see them during the 1, 3, or 7 days that the city pass is valid? Will seeing that many places have you running around more than you would like? You have to travel the way you want.