What is Flat Design?

Taken from wikipedia:

“Flat design is a style of interface design without any stylistic elements that give the illusion of three dimensions (such as excessive use of drop shadows, gradients or textures) and is focused on a minimalist use of simple elements, typography and flat colors.”

Flat website designs tend to be clean and clear, with lots of blank space and large typeface. Modern trends in website design place an emphasis on readability and usability instead of aesthetic complexity.

Check out this link to see some examples.