Hi, Craig! You are correct that busses run approximately every 20 minutes. What this means is that you should not have to wait longer than 20 minutes between busses. There is not a schedule that states, for example, that a "Magic Kingdom bus will arrive at 9:20."
When the bus arrives at your stop, it will be clearly marked of its destination (Epcot, Animal Kingdom, Downtown Disney, etc.) Just wait for the bus going to your destination and off you go!
A website you might want to look at is
www.ourlaughingplace.com. Go to the bottom of the page to the travel wizard, and it will calculate approximate travel times for you.
Have fun planning your Disney vacation!