Each stop is clearly signed and the buses have the destination on the front so you can't go wrong
Buses run every 20 minutes approx. Although at peak times (early morning and at park closing) this time may be shorter because Disney can re-direct spare buses to wherever they are needed most - very efficient
You can also get buses between parks if you want to hop from park to park

If you are in MGM or Epcot you can also catch a Friendship boat that will call at the Boardwalk, Swan/Dolphin and Yacht/Beach Club resorts.
There are no buses that go from resort to resort though (unless you count the resorts that share the same bus, like the Yacht/Beach Club resorts and the All Star resorts).
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A few buses will visit more than one park - I think the MGM bus also stops at Blizzard beach. All the other buses are direct from AKL to the parks.
The bus stops at AKL can be found by leaving out of the front entrance and turning right. (or you can take a short cut through the shop

).
Each bus stop at AKL is signed to which park it covers
