Although the “flat design” concept is not new, being successfully used many years ago within design projects, it looks like this year it made a comeback. And what a comeback! Facebook and Google already followed the trend and improved their product designs with flat elements. Microsoft increased the trend popularity by its Windows 8 releases and many other websites are successfully implementing elements which define a flat design.
There are a couple of principles which can make a design to be called flat: simple UI elements without any shadows or gradients, color and typography focus, accent on content hierarchy. But I don`t see reasons to explain them. Better, take a look at these awesome examples of websites which are successfully showcasing the flat web design principles.