Increasingly on the Internet, websites are personalizing themselves to suit our interests. We all see this happening at Amazon, where if you order a book, Amazon will send you the next book. We see it happening in Netflix, but it's also happening in a bunch of places where it's much less visible.
For example, on Google, most people assume that if you search for BP, you'll get one set of results that are the consensus set of results in Google. Actually, that isn't true anymore. Since Dec. 4, 2009, Google has been personalized for everyone. So when I had two friends this spring Google "BP," one of them got a set of links that was about investment opportunities in BP. The other one got information about the oil spill. Presumably that was based on the kinds of searches that they had done in the past. If you have Google doing that, and you have Yahoo doing that, and you have Facebook doing that, and you have all of the top sites on the Web customizing themselves to you, then your information environment starts to look very different from anyone else's. And that's what I'm calling the "filter bubble": that personal ecosystem of information that's been catered by these algorithms to who they think you are.