Flat design is a technique that uses simple effects – or lack thereof – to create a design scheme that does not include three-dimensional attributes. Effects such as drop shadows, bevels, embossing and gradients are not used in flat design projects.
Some call the look of flat design simple, although it can be quite complex. The look itself is simple, direct and user-friendly, making it an increasingly popular option for mobile user interfaces as well as trendy web design.
Via Hans
Flat web design obviously has benefits for both web and mobile users. Grid based layouts are scalable and easy to choreograph, modular graphics maximize both visual surfaces and interactive areas, and distinct typography is readable on small screens. On the other hand, it's also a challenge because flat web design is (almost) a completely web based design language. It feels like Internet grew some confidence as a virtual reality, and decided to break free from the limitations of our natural environment. So in contrast to skeuomorphism, it lacks references outside the digital domain like textures, buttons, depths and shadows. And if the design patterns are not mapped to the users' previous knowledge intuition will be lost. Consequently, I'm waiting for the world to decide if it's is just a trend that will pass by or if it's a new web standard. Is it time to let go of old notions, accept the digital medium and base graphics on design languages like flat web design?