Apple has dramatically revamped the look and feel of the iOS user interface for its mobile devices.

"It's the biggest change to iOS since the iPhone," Apple CEO Tim Cook said Monday during the keynote at WWDC, where the company introduced the new version of the software that runs its iPhones and iPads.

For the latest version of iOS, called iOS 7, the company revamped the user interface, giving it a new, "flat" design. What does that mean? For one, Apple has taken the edge off the icons that populate its home screen. And some of the icons now sport a new logo and color.

"Design defines so much of our experience," Apple's Jony Ives said in a video touting the new design. "I think there is a profound and enduring beauty in simplicity, in clarity, in efficiency. True simplicity is derived from so much more than just the absence of clutter and ornamentation. It's about bringing order to complexity."

Everything has been revamped, including the icons, buttons, and color schemes. The screen will take on a 3D appearance when you move your device around. There's also a way to see a preview of an app in what looks like a stack of cards when you're multitasking.