I'm no expert, but our family has been twice in the last two years. Other posters feel free to correct me if I am wrong...
It basically works as a way to skip long lines by "reserving" a spot in line. There are only fast passes available for certain rides and you can only get a fast pass for each ticket every two hours (the exception is if your fp time to return is less than two hours away). Here is how I worked it:
1) determine which rides are fp by going to Disney website
2)decide which rides my family thought were must dos
3) then I created a touring plan based on the above
When we got to the park, we designated a fast pass runner to go get our fastpasses and they met us at a different ride in standby. This worked really well for us and we were able to ride lots of rides during the week before Easter, which means it was at capacity.