We're looking at spending a day at Disney World in early December but the size of the place is mindboggling - I read that it contains four theme parks.
As you may have already found out on the Disney site, there are various "park hopper" tickets that let you go to more than one park on the same day.
We spent an entire week at Disney World in 2009 and didn't see everything. That was staying at a Disney hotel close to Epcot, with early admission and late exit to/from the parks (only available to Disney-hotel guests) every day. We had park hopper tickets, but rarely used the multi-park feature more than once a day -- usually devoted most of the day to one location, and perhaps returned to another park after dinner to take advantage of the late exit privilege.
If you have been to Disneyland in California before, then I would concentrate on Epcot and Hollywood Studios since they will be different from your previous experience. They are also relatively close together. If you have never been to a Disney theme park, then by all means just concentrate on the Magic Kingdom park if you have only one day, since that park is the most Disneyesque.
Keep in mind that the overall Disney World property, including all of the various parks, water attractions and Disney-run hotels is huge -- twice the size of Manhattan Island. Most of the parks are several miles apart and linked by a 4-lane highway. You could fit almost 170 Disneylands (180 acres) inside of Disney World (30,500 acres). You can't just hop from one park to another in a few minutes.
In addition to automobile parking at each location, there are free shuttle buses running between each of the parks. There is also water transportation between Epcot and Hollywood Studios, and the Magic Kingdom is linked to Epcot via a monorail.
For those who come across this question, "Disney World" (or Walt Disney World) is the name of a resort in Florida. It contains four theme parks:
Magic Kingdom - the main park which contains the Cinderella Castle, Space Mountain, Pirates of the Caribbean etc.
Epcot - a "prototype city of tomorrow" containing Spaceship Earth, various pavillions and attractions based on different countries etc.
Animal Kingdom, containing various animal attractions and the new Avatar area
Disney's Hollywood Studios, with the recent Star Wars and Toy Story areas, plus more based on the movies
The Disney World resort also contains many hotels, water parks, a shopping precinct, and lots more. Many people will mistake the Magic Kingdom park for being "Disney World", as the park is mostly identical to the "Disneyland" park in California (itself a part of the Disneyland Resort, containing two parks, hotels, and a shopping precinct).
You can buy individual admissions to the parks, or various tickets giving you access to all the parks for varying periods of time.
As with most large theme parks, it is not advised to attempt to visit more than one in a day as you would likely be spread thin across the attractions.
Getting between the parks can be difficult - Disney World is a large place. If you are staying on-site, Disney do provide different transportation options, but it is by far the best if you have your own (eg a rental car).
