Apple announced during the WWDC in June that OS X 10.10, a.k.a. Yosemite, would be released this Fall. Demos were given of Yosemite’s new interface and features, and since then Apple has posted three preview versions to OS X Beta Program members. https://appleseed.apple.com/sp/betaprogram/
I’ve installed OS X 10.10 Developer Preview 3 on a couple of my lab Macs. Like past OS X releases, Yosemite has a lot of new eye candy, plus Apple had made some substantial design changes to very familiar Mac user interface elements (the new Finder icon gives me a whole new reason for wanting to hide the Dock). Overall, it’s looking more and more like iOS, which I guess is the point. iOS 8 will be released about the same time that OS X 10.10 is, and the two are being marketed as offering “a completely new relationship between your Mac and iOS device”.