This was posted yesterday on the Google blog.
http://googleblog.blogspot.com/2010/06/our-new-search-index-caffeine.html
So why did we build a new search indexing system? Content on the web is blossoming. It's growing not just in size and numbers but with the advent of video, images, news and real-time updates, the average webpage is richer and more complex. In addition, people's expectations for search are higher than they used to be. Searchers want to find the latest relevant content and publishers expect to be found the instant they publish.
To keep up with the evolution of the web and to meet rising user expectations, we've built Caffeine. The image below illustrates how our old indexing system worked compared to Caffeine:
Our old index had several layers, some of which were refreshed at a faster rate than others; the main layer would update every couple of weeks. To refresh a layer of the old index, we would analyze the entire web, which meant there was a significant delay between when we found a page and made it available to you.
http://googleblog.blogspot.com/2010/06/our-new-search-index-caffeine.html