Congratulations to Raya for turning Omnibus Orange and to me for turning Skyliner Purple!
The transportation system that most WDW guests are familiar with is the resort bus system, which connects the parks, the resorts, Disney Springs, and the water parks. Unless other transportation is offered (such as monorail or boat), you can take the buses to and from resorts and parks, to and from resorts and Disney Springs, and after 4 p.m., from parks to Disney Springs (pre-COVID, anyway). If you want to go resort-to-resort, though, you'll have to go to a park or Disney Springs and transfer.
WDW buses are ADA accessible and can carry two wheelchairs or scooters. The capacity of regular buses is about 70, but the articulated buses can hold over 130! Buses run from resorts from about 45 minutes to one hour before park open until about an hour after park close. And they run from Disney Springs until 2 a.m.!
Riding a bus from a WDW resort to MK lets (or forces) you to skip both the ferry and the monorail from the TTC. Starting in the late 1980s, the resort buses began to stop just to the side of the MK's gates instead of at the TTC. The bus stop area has since been expanded to accommodate the increasing number of resorts, and today no WDW resort buses use the TTC. (Other on-property resorts have their own buses that do still use the TTC. That includes Shades of Green, and since last fall, also the Swan and Dolphin.)
During COVID times, WDW resorts have been restricting visitors who don't have a reservation or a dining reservation from entering by car, so the bus system provides a way to resort hop.
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The other familiar WDW bus is Disney's Magical Express, a complimentary service that will transport you between MCO (Orlando airport) and the WDW resorts. Sadly, in January 2021, Disney announced that Magical Express would not be provided for trips beginning on or after January 1, 2022. But the service is run by Mears Transportation, which already has announced it will be providing bus service to replace DME (of course, though, for a price).