We've used it at both Disneyworld and Disneyland. Basically fast pass is available at the most popular rides at each Disney park. It's free! Immediately upon entering the park go to one of these rides and go to the Fast Pass kiosk and slide your ticket thru-A fass Pass for that ride will come out with a time to visit that ride. It'll be like a 1/2 hour window of time. Go enjoy yourself and go on other rides in the park or a parade or whatever but be sure to return during that time window . When you do you get on the Fass pass line and you'll be on your ride in no time!! Once you get off the ride you can go get another Fast Pass. You cannot get more than one fast pass at a time and the next time you can get one is after you used the first or after a certain amount of time.
Hope this helps!