ssatterly:
When you approach a FastPass attraction, you will be presented with a LOT of information. You will see a line labeled "Standby", along with a clock indicating the approximate wait time. The Standby line is for folks who do not want to use FastPass. So, if you approach the line for Space Mountain and the Standby clock reads 10 minutes, you should only have to stand in that line for about 10 minutes before you will board a car.
You will also see a separate section for Fastpass. Listed above the FastPass queue will be the return time IF you get a FastPass ticket immediately.
In other words, if you approach Space Mountain at 10am, and the FastPass Return Time is listed as 11:50 to 12:40 (it's always a 50-minute window), you know exactly when you will have to be back in the area to use a FastPass ticket obtained now. If that time works for you, there are usually 6-8 FastPass ticket machines. Stick your park pass in any one (similar to an ATM) and you'll get back your park pass along with a FastPass.
If the time listed doesn't work for you (will have left the park by then, eating a meal, parade, etc.), then just don't get a FastPass.
As crzy4mk said, you can hold more than one FastPass at a time. Each time you get a FastPass, it will not only tell you your return time for that attraction, but it will also tell you the time you can get your NEXT FastPass. The next FP can be for the same attraction or an entirely different attraction--doesn't matter a bit.
If you have questions once you arrive at the park, there are almost always CMs overseeing the FastPass ticket machines at each attraction. They should be able to answer your questions.