From this article:

Glitch, at its most basic, is a 2D Flash, Web-based, social MMO with a heavy puzzle-solving component. Or, as the team likes to call it, a "collaborative sim." Its back story centers on a great but very dark future, and a group of scientists who discover a path back in time to create the optimistic future everyone wants.

"The whole world was spun out of the imagination of 11 great giants," Butterfield, who often speaks in a soft, hard-to-hear voice, said. "So you have to go back into the past, into the world of the giants' imaginations and grow...the number of things in the world, grow it in terms of physical dimensions, to make sure the future actually happens. So all the game play takes place in the past inside the world of the giants' imagination."

Practically speaking, Glitch is a social game about learning how to find and nurture resources, identify and build community, and proselytize to those around you. It's not about epic, bloody battles. "Rather than you and me fighting each other with swords," Butterfield explained, "it could be you and me having rival religious factions battling each other for converts."

Along the way, players level up by completing quests, gaining skills, growing all manner of things, and making their way through a sometimes Mario-esque world of different artistic styles, each of which can be thought of as being inside an individual giant's memories.