Instead of a drive chain made up of metal linkages and up to 3,000 individual parts, chainless bikes utilize a carbon belt similar to the drive belt on motorcycles and snowmobiles. (They're manufactured by the same Denver-based company that makes drive belts for Harley-Davidson.)
The carbon belt has a number of advantages over a traditional chain: lighter weight (80 grams vs. 300-400 grams), longer lasting (3 years vs. 1 year), minimal stretch, and lower maintenance (no lube required!). Belt drive systems are virtually silent and rarely catch pant cuffs, making them a good choice for commuters.
The carbon belt is a great solution for single speed bike like Trek's bike design with the urban commuter in mind, but it not yet ready for use on multiple speed bikes.
Blog Site for Trek's District