A video posted on Reddit shows multiple screenshots of Android L up against Android Kit Kat. Android L was released at Google I/O this year, and is Google's next version of Android. It is a major UI overhaul from Android Kit Kat and prior. It features Google's new Material Design guidelines, and has 5000 new APIs. Right now, Android L is in a developer preview state if you want to try it out on a Nexus 5 or Nexus 7. The developer preview isn't exactly daily driver worthy, but it will give developers a good idea of what to expect when Android L is released to the masses this fall. The Developer Preview is here to let developers play around with it and get their apps ready for Android L when it ships this fall with a new Nexus device. Which Google did confirm that the Nexus program isn't going away.
In the video, Android L's UI changes become apparent when contrasted against Android Kit Kat, which was released last fall and brought some UI changes as well when compared to Android 4.3 Jelly Bean ahead of that. Some of the 100 screenshots included in the video compare the calendar, gallery, maps, contacts and others. You'll want to check out the video at the bottom of this post though.
The differences are stark. If you aren't able to flash Android L or emulate it on your computer, be sure to watch the video.