And Alexis Madrigal writes about it, in the most general terms, at the Atlantic. It was started by John Giannandrea, Danny Hillis, and Robert Cook as Metaweb in 2005 and Google subsequently bought it five years later and enlarged it. A lot. And now it's helping you, at the right-side of a Google search result.
What did Google bring to the acquisition, aside from money? Data, of course, of a very specific kind. Before, they were just guessing at what people might want to know (cheese, rivers, highways, etc). With Google's search data, they *know* what users are after, so they can go about finding and making that information available.With Google's help, their database has grown rapidly to over 500 million items objects. That's orders of magnitude larger than previous attempts to educate artificial intelligences like the Cyc project out of the University of Texas. (Though it should be noted that Cyc has some capabilities that the Knowledge Graph does not.)In the end, what is most significant to Giannandrea is that "we're taking a baby step in teaching all our computers at Google something about our human world." As for what comes next, he can't say, but the idea is that it will become a resource that all Google developers can call on, the core of common sense at the center of Google's vast web.
Of course I don't know what's under the hood, but I'd be surprised if it wasn't based on ideas first floated back in the late 1960s and that flourished through the mid 1980s, when they were all but abandoned in favor of entirely different techniques and somewhat different problems. The above-mentioned Cyc project is from that era.
I wonder what the ontology looks like?