I think,
The Wikipedia explains that a tumblelog is a variation on traditional blogging that favors very short, mixed media posts with little or no commentary. In essence it's a lightning-fast way to publish the stuff you run across on the web every day, and the layout includes large text, photos, links, and video frames. A really great example of a well-done tumblelog is projectionist.
Tumblr is a new service that publishes tumblelogs.