Such a simple question with such a variety of answers.
Simply, the Cube is a draft format where you get to play designer and developer. You get to choose your metagame. You choose all of the cards in it. You designate powerful strategies, archetypes, and you decide which ‘foils’ to those strategies you would like to include.
The Cube’s exact origins are unknown. But for myself, it began in June 2005 when I read an incredible article detailing the format. The gist of a cube is this: The Cube is the best cards in Magic, stuffed together, to create incredibly fun experiences. How you create or mold those experiences is up to you. This website is here to help you figure out what my Cube looks like, what others look like (via the forums), and the possibilities the format contains.
Remember: If you’re not having fun, then you’re doing it wrong. Take a look around, ask questions, and good luck!
For those looking for a How To Guide on creating a cube, take a look at this blog post on how to get started.
Here is an almost hour-long talk I gave on the Cube and its intricacies on the 2009 Magic Cruise in February 2009:
Watch The Magic Show #61, Gleaming the Cube, to learn more about the format:
Tom is currently an R&D Intern at Wizards of the Coast and has been a big proponent for the format. His Handoff Design Document can provide a good starting point for those looking to configure exactly what the colors “should do” in a cube.