It's a new way to structure and query data -- essentially a new type of database, a variation of a graph database -- and the result is that anyone can use it and model their data without realizing that's what they're doing. I wish I could describe it in a sentence or two, but the upshot is that while Google is great at finding text in web pages, a business or a schedule of events doesn't want to be a web page -- it wants to be found. So by allowing people to structure their own information, you get a different type of web experience: show all babysitters near me who charge less than $6/hour, or send a message to all co-workers who play squash. Things like that.