As with all OS updates, Apple promises increased speed and battery life efficiency and the update will overhaul the aesthetics to better match that of iOS 7 and its cleaner design.
In addition, the OS will have an improved notification system, including inline replies, support for multiple monitor and full screen applications, a complete refresh of Finder and a new "iCloud Keychain" to better save your passwords.
Many of the apps have been updated, as well, some adding new functionality.
The update is available to everyone running OS X 10.6 or higher and now leaves Microsoft as the only company that charges for their operating system.