The Reedy Creek Improvement District is the entity through which Disney is, in most matters, their own government. Walt Disney World is physically located in two different Florida counties. The majority of the property is in Orange County, however, some of the southern parts of the property are in Osceola County.
The fact that the property wasn't in a single county led to some difficulties during initial development in the sixties. This meant there were two sets of building codes and two different local governments to deal with. Disney petitioned the state of Florida, which created the Reedy Creek Improvement District. RCID allowed "Disney" to create their own building codes, and also removed the burden for infrastructure requirements such as fire departments, water, sewer, and road improvements from Orange and Osceola Counties.
RCID doesn't have its own police, though, which is why you'll see Orange County and Osceola Country Sheriffs in the parks. RCID also doesn't levy taxes. Sales and hotel taxes still go to the county in which the sale takes place.