Apple admitted it had trouble naming the next generation of OS X, but it picked a name devoid of cats so that it'll continue on for years to come. OS X Mavericks 10.9 is what it's called, Apple's impressive next-generation Mac operating system slated for a fall release. Little has been done done in terms of user interface design, however, a lot has been focused on improvements and more than 200 new features that some more notably intelligently help prolong battery life thru smart power consumption and power-saving technologies like App Nap that run seamlessly behind the scenes as you work to reduce CPU load by slowing down apps that are hidden in the background when you are focusing on something else.
Among a sea of new features and enhancements which some include power users in mind, OS X Mavericks supports up to 4K display resolution and a featured called Multiple Displays allows the system to operate individually across multiple external displays whilst retaining menu bars and docks on each screen instead of relying on one primary display interface to handle different tasks. It also has deeper iOS app integration than Lion has with apps such as iBooks and Maps now available on a desktop level. Your iOS apps will sync with your OS X apps, and Finder has been enhanced with organization tabs and tagging for quicker more specific search results.
Apple got rid of Scott Forstall's old school design choices which included wood, leather and felt elements in app interfaces. The newly designed Calendar app for example is now more simplistic look to coincide with the rest of the stock apps. There's a lot more to discover, and you can learn more about OS X Mavericks on Apple's detailed OS X Mavericks overview page.
