Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second OS X 10.10.3 Yosemite Beta to Developers
The new beta, build 14D87h, is available for registered developers through the Software Update mechanism in the Mac App Store and through the Mac Dev Center.
OS X 10.10.3 brings several new features to Yosemite, including the new Photos for OS X app. Photos is designed to serve as a replacement for both iPhoto and Aperture, and is on track to see an early 2015 launch. Early reviews have suggested that while Photos is a suitable replacement for iPhoto, with more advanced tools and performance optimizations, it may leave professional users disappointed, at least in its initial incarnation.
The first beta of OS X 10.10.3 also brought a new emoji picker that consolidates emoji into a single page with clear labels, and support for Google 2-step verification when setting up accounts in System Preferences.
Update: Today's 10.10.3 update introduces new diversified emoji options and new skin tone modifiers for emoji, as hinted at in the previous 10.10.3 beta. There are also new country flags and updated emoji for the iPhone, iMac, and Apple Watch.