A few them would be the replacement of iChat with Messages, iCal with Calendar and Address Book for Contacts. With OS X Mountain Lion, users will have better synchronization of information across their Apple devices - thanks to iCloud. Plus, other third-party cloud services gets integrated as well.
However, the Mountain Lion update is not yet shipped. Users can simply view its new features in Apple's website or download and install a beta version for those with developer accounts.
After this latest OS X, Apple plans to make the software updates on a regular yearly schedule, much like how they do it with the iOS.
And just in case you're wondering, Apple did not have an announcement event for this news. Perhaps they're busy preparing for the Apple press conference in March where the much rumored new iPad 3 will be announced.