Basically it is avalaible for everyone, you would take your park ticket and put it into the fastpass machine (which is located at the attraction entrance). You will then be issued a pass that has a return time window written on it (it is an hour long window which you can return anytime within the window). You will get your ticket back and your fastpass (each person in the party must us their own ticket to get a fastpass). Then you can go and do other rides or get a snack or whatever. When the time comes that is printed on your Fastpass ticket you go back to the attraction and enter through the fastpass que instead of the stand-by line. You will still have a short wait in the fastpass line, but it is usually 10 minutes or less (if everything is working properly.
The bottom of the fastpass ticket will also tell you when another fastpass will be avaliable for you. In theory you can only have one fastpass at a time, but in reality the posibillity is there that you could have more than one. If you try and get another fastpass before the time printed on the current one, you will get a ticket that tells you you already have a fastpass and it will again tell you when the next one would be avalaible.
Here is a list of rides that have fastpass avalaible
Animal Kingdom:
Dinosaur, It's Tough to be a Bug, Kali River Rapids, Kilimanjaro Safaris, Primeval Whirl
Disney-MGM Studios:
Indiana Jones Epic Stunt Spectacular, Rock n Roller Coaster, Star Tours,
Tower of Terror, Voyage of the Little Mermaid
Epcot:
Honey, I Shrunk The Audience, Living with the Land, Maelstrom
Mission: Space, Test Track
Magic Kingdom:
Thunder Mountain, Buzz Lightyear, Haunted Mansion, Jungle Cruise,
Winnie the Pooh, Mickey's PhilharMagic, Peter Pan, Space Mountain,
Splash Mountain