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.