The latest, greatest version of the Mac OS 10 Yosemite (version 10.10.3) which features the new Photos App that replaces iPhoto and is being pitched as a substiture to the discontinued Aperture application on notebooks and desktops.
Other improvements in this update are 300 new emoji, among them ones with different skin tones, and the addition of Spotlight suggestions for the Look Up panel. Various bugfixes for Wi-Fi performance and captive network compatibility, Bluetooth devices disconnecting, and the reliability of screen sharing. Safari should have better stability and security, and no longer save website favicon URLs used in Private Browsing mode.
OS X 10.10.3 can be downloaded via the Updates tab in the Mac App Store app.