EPCOT is quite a difficult concept to explain if you have no prior knowledge of the park's origins, but it is also a fascinating history and progressive view for the time. The park, originally, was not a park at all - it was a vast community, to be the central hub of the Walt Disney World. It would boast housing complexes, hotels, and shopping districts, would have its own government, and would be ever-changing as new technologies developed. Upon Walt's death in 1966, the project almost died with him.
The Disney Board was weary about starting this venture without its visionary leader, so Walt's brother, Roy, came out of retirement to work on the project. While the idea of the central living community was scrapped, the underlying concepts of the EPCOT were put into place as the theme park, which opened in 1982.
From the opening day map above, you can see that EPCOT Center was split into two different sections - Future World, which offered a look into different areas of science and technology, and World Showcase, which offered looks at various cultures from around the world (much like the World's Fairs popular in the 60's).
When EPCOT Center opened, not all of the pavilions were completed - Horizons and The Living Seas in Future World were not yet complete, nor was Morocco in World Showcase.
The idea for the pavilions came from the World's Fairs of the late 60's (including the one in New York where Walt Disney introduced the world to several new technologies, like audio-animatronics, and several attractions that featured them, like The Enchanted Tiki Room and the Carousel of Progress). From my understanding, World's Fairs were set up to where each country (or region) had its own pavilion, where it would showcase films about the country or offer shops with merchandise from each locale. The idea with the pavilions at EPCOT Center harp back to the original concept for the city - each pavilion would be (and still is) sponsored by a major corporation that funds it, and helps build the technology that goes into it. This is most prominent in Future World.
Spaceship Earth, Communicore, the Universe of Energy, Horizons, World of Motion, Journey into Imagination, The Land, and The Living Seas pavilions all had corporate sponsors, who paid for rights to have their logos included on the buildings. The countries of World Showcase, alternately, were funded in part by the countries they represent.
By 1991, you can see there have been a lot of additions to the park - in Future World, Horizons was complete, The Living Seas was open, and the Wonders of Life pavilion had also been built.
World Showcase saw even more changes in those first several years. The Morocco Pavilion was completed, and the Norway Pavilion was added. Also added was the International Gateway - this is the waterway that led to Walt Disney's World newest theme park for the time, Disney-MGM Studios.
In 1994, the name of the park was changed to "Epcot '94", and subsequently, "Epcot '95" the next year, bringing back memories of the old World's Fairs. In 1996, the name was simply changed to "Epcot", which it remains today.
The 1998 edition of the map brought the artwork we are now accustomed to, and as you can see, the World of Motion pavilion has been renamed Test Track. This really paved the way for the other pavilions to become attractions in themselves, and drop their original EPCOT Center names. Communicore had also been changed to Innoventions, as communication had been getting so broad, that the focus shifted to invention and innovation.
By 2003, other changes to Future World had taken place. The Horizons pavilion had been demolished to make way for a new attraction. The Journey into Imagination Pavilion was simply called "Imagination!", and several of the shows and attractions within the other pavilions had changed with the times.
By 2008, even more changes happened. The attraction that replaced Horizons, called "Mission: Space" had opened to much acclaim. The Wonders of Life pavilion closed its doors soon after Mission: Space opened, and The Living Seas had been taken over by Nemo and his Friends. The Land pavilion had received a major facelift, and had added the attraction "Soarin'", which was originally at Disney's California Adventure. Not to be ignored, several of the attractions and rides in World Showcase received facelifts as well.
So although the concept has drifted from Walt's Community of Tomorrow, the essence of Epcot remains. It is an ever-changing, always-evolving, constantly progressive park. Personally, it's my favorite, and I can't wait to see what tomorrow holds!