Paris' METRO will be familiar to you if you've ever traveled on any metropolitan train system (except maybe Boston's).  It's easier to use, in fact, than many.
There are three train systems that serve Paris:
If you will be visiting Paris, don't drive; use the trains and buses; traffic inside Paris is indescribable both for its volume and ferocity, and unless you are very good at French, the time it takes to translate the signs will prove hurtful if not fatal.  Beside that, there's no place to park, and the trains will take you within 500 meters of wherever you wish to go.
The rail transport system is divided into fare rings, 1 through 5, with "1" being the vicinity of Notre Dame and "5" covering the outlying areas including the airports.  Almost everything you will want to see in Paris will lie within the three inner rings.  Buy separate tickets for anything else.  Warning: know which ring your trip will take you to and buy the appropriate ticket beforehand if necessary. There is a huge fine for trying to exit outside your fare zone.  You can get a "Paris Visite" ticket at METRO stations and Visitors' Bureaus.  They can be had for 1-, 3-, or 5-days, and for 1-, 2-, 3-, 4-, or 5-zones.  These tickets are good on the bus system, which is also part of RATP.  If I knew then what I know now, I would have bought two sets of 5-day 3-zone tickets, purchased round-trip tickets for CDG-Downtown, and bought individual tickets for everything else.  It would have saved us $50 or $60.
On board the trains, which come in all shapes, sizes, ages, and conditions, you will find full-system maps as well as "strip maps" showing only the route you are riding.  Of course they show which stations allow connections to other routes and other lines.  It's too simple.  If you can't successfully navigate through the Paris METRO, you probably shouldn't be left unattended.
The stations are cleanest in the outskirts and dirtiest the more central they are.  Some of them are so dirty and covered with litter that even New Yorkers are shocked.  Virtually none of the stations are what we would call "handicap accessible".  If you are unable to climb long flights of stairs in either direction, the METRO is not for you.
If you're familiar with, for example, NYC's subway system you might expect that all METRO stations will be pretty much alike and you would be in for a surprise.  Trip over to parislogue.com to see a sampling.