When folks think of Walt Disney World, they are usually thinking of Magic Kingdom. Magic Kingdom is the park with Cinderella's Castle as the centerpiece, and it's what us adults remember seeing as a kid on Sunday night's Magical World of Disney shows (well, okay, maybe that was DisneyLAND in California, but back then they looked pretty similar). Magic Kingdom is the first park opened at Walt Disney World, so for that first decade, the terms were synonymous. When Epcot opened in the early 80's, it became important to start calling that first park Magic Kingdom, using the name Walt Disney World to denote the entire place.
Walt Disney World consists of four major parks - Magic Kingdom, Epcot, Disney-MGM, and Animal Kingdom. There are also some minor attractions, including the two water parks - Blizzard Beach and Typhoon Lagoon. The area known as Downtown Disney consists of shops and restaurants, a permanent Cirque du Soliel facility, a more-than-nightclub complex called Paradise Island, a bigger-than-you-thought arcade/entertainment complex called Disney Quest, and a movie theater. There are something like 14 Disney Resorts within the whole "world," offering thousands of hotel rooms nightly. Within the Downtown Disney area are hotels with are not really officially Disney, but are called Good Neighbor Hotels.
Within the "world" you'll find a sophisticated bus transportation system. There are three major lakes within the World (one defining the Magic Kingdom area, one defining the Epcot Resorts area, and one as the focal point at Downtown Disney. Boat transporation is offered on these lakes. Also, there's a monorail system that connects the Magic Kingdom to its resorts (Grand Floridian, Contemporary Resort, and Polynesian) as well to its parking lot (aka Ticketing Center). There's a monorail that connects Epcot to the Magic Kingdom Ticketing Center.
If you are planning your very first trip to Walt Disney World, let me sum it up for you: BIG. This is not your typical Six Flags amusement park. This is not akin to a Sunday visit to the zoo. Walt Disney World makes the Smithsonian look like a mini mall. That said, your goal should be to visit WDW and enjoy yourself. Your goal is NOT to see and do everything. Your goal is not to outshine your neighbors who were there last May for a week and whined at the Block Party that they didn't do it all.
The best strategy for first timers is to stay onsite at a Disney Resort, stay for at least 4 nights (but 6 or 7 nights is even better), and utilize the Disney transportation system. Plan your days in advance, and plan manageable days. Allow for some down time, including pool time and special dining. And get it out of your mind that you're going to see it all!!!
