OS X 10.8 “Mountain Lion” is available as a direct upgrade for any users of Mac OS X 10.6 “Snow Leopard” or Mac OS X 10.7 “Lion”. Users of operating systems prior to that will have to purchase either the Snow Leopard DVD or the Lion USB upgrade media and install that before they can upgrade to Mountain Lion. One installation will work on as many eligible Mac computers as you own.
OS X “Mountain Lion” brings numerous major enhancements, including Game Center support and client, Notification Center, an updated IM client with iMessage support, Dictation support anywhere there’s text inputs, enhanced iCloud support, a slightly redesigned Dock, new Reminders and Notes applications, a new “Power Nap” feature which allows your Mac to update even while asleep, OS wide social support including Twitter and Facebook, Air Play support, an all new security system entitled Gapekeeper which checks 3rd party applications not purchased on the App Store for security risks, and more.
You can purchase OS X 10.8 “Mountain Lion” from the Mac App Store now from here.