Disney transportation is an efficient way to get around on property, but there are a few quirks. Here are the ones you'll need to know about:
You can't take a bus from Downtown Disney to the parks, just to the resorts. This is to discourage people from parking at Downtown Disney--for free--rather than at the parks.
There are no buses running to and from the Polynesian, the Grand Floridian, and the Contemporary Resort to Epcot. You'll need to take the resort monorail to the Ticket and Transportation Center and transfer to the Epcot monorail. Tip: If you're at Epcot during closing and the line for the monorail is too long, you'll pay around $10 for a cab from Epcot back to your monorail resort.
You can take a bus from the Ticket and Transportation Center (TTC) to your resort, but you can't take a bus from your resort to the TTC.
Buses do not run from resort to resort. This poses the biggest problem when you're eating at one resort and staying at another, particularly in the early morning. Instead, you'll go to the nearest park and change from there. For example, if you're staying at the All Stars but eating dinner at the Contemporary, you'll take a bus to the Magic Kingdom and then walk or take the monorail to the Contemporary.
The Magic Kingdom is serviced by two monorails: The resort monorail and the express monorail. Both stop at the Magic Kingdom and the TTC, but only the resort monorail stops at the resorts. You don't have to be a resort guest to ride the resort monorail.
You can rent a car on Disney property, but it's often more expensive than renting at the airport. This is of course different from the norm; in most cities, taxes and fees make airport rentals higher than in-town rentals.
If you'd like to hear more about Disney transportation, we discuss Magical Express and the various ways of getting around property on the latest edition of Mouse Chat. You can listen here and on iTunes.