Yikes...Ok, here is my crack at it:
Fastpasses allow you to return to an attraction in a one hour window (called the fastpass return time) and bypass the standby line. The fastpass line is MUCH shorter but it is not a front of the line pass.
There are fastpasses available for both rides and shows...ignore the shows fastpassses for now, they do not affect your ability to get a ride fastpass. You use your entrance ticket to pull a fastpasss at the kiosk at each attraction. It will have the return time window on your fastpass for when you need to come back to that ride. You can not use it before or after that one hour window.
When you pull a fastpass, you can only pull another fastpass two hours after you pulled your previous one OR at the return time of your current fastpass, whichever is EARLIER. Example: You pull a fastpass at 1:00pm for Space Mountain an it has a return time of 6:00pm, you are eligible for another fastpass at 3:00pm (two hours after pulling the Space Mountain pass at 1:00pm). If you pull that Space Mountain Fastpass at 1:00pm and the return time is say 2:00pm, your next fastpass is eligible to be pulled at 2:00pm.
Disneyland and DCA are NOT connected meaning that having a faspass from DL does not effect your timing to pull a fatpass from DCA. You can pull a faspass for Tower of Terror then immediately walk into DL and pull a fastpass for Space Mountain regardless of your Tower return time.
Shows are also disconnected from the attraction time restraints, meaning if you get a fastpass for World of Color, it does not effect your ability to pull a fastpass for an attraction.
If planned out well, your ride volume will increase dramatically. I tale the approach that I should NEVER be without a fastpass in my pocket for something if I am eligible to pull one at any given time.