More details are available from the Google blog:
A typical newspaper article leads with the most important and interesting news, and follows with additional information of decreasing importance. Information from prior coverage is often repeated with each new online article, and the same article is presented to everyone regardless of whether they already read it. Living Stories try a different approach that plays to certain unique advantages of online publishing. They unify coverage on a single, dynamic page with a consistent URL. They organize information by developments in the story. They call your attention to changes in the story since you last viewed it so you can easily find the new material. Through a succinct summary of the whole story and regular updates, they offer a different online approach to balancing the overview with depth and context.