If you need to purchase Mac OS X 10.7 Lion, you may order it from this page.
The most current version of OS X is OS X 10.9 Mavericks. To learn more, please click here.  
What do you receive: An email with a content code for the Mac App Store.  
Note: Content codes are usually delivered within 1 business day but may occasionally take longer. The use of content codes and redeemed software is subject to the terms and conditions of the Mac App Store (http://www.apple.com/legal/itunes/ww/) and the OS X Lion End User License Agreement (http://www.apple.com/legal/sla/). Some features require an Apple ID; terms apply. Some features require a compatible Internet service provider; fees may apply. Some Lion features have additional hardware requirements. These requirements may be found at http://support.apple.com/kb/HT5444. System Requirements    
An Intel Core 2 Duo, i3, Core i5, Core i7 or Xenon processor
Mac OS X Snow Leopard v10.6.6 or later (v10.6.8 recommended)  7 GB free hard drive space  2 GB RAM

OS X Mountain Lion is available now as a download from the Mac App Store. Just click the Mac App Store icon in your dock, purchase Mountain Lion, and follow the onscreen instructions to install it. Mountain Lion will not only make your Mac work better, it also makes your Mac work even better with your iPhone, iPad, and iPod touch. Because it comes with iCloud, your mail, calendars, contacts, reminders, documents, notes, and more stay up to date on your Mac and iOS devices — automatically and effortlessly. Messages with iMessage makes it possible to send messages to anyone on an iPhone, iPad, or iPod touch running iOS 5. Notification Center keeps you on top of your email, messages, software updates, and calendar alerts in two ways. Notifications appear in the same spot on your desktop, and you can see all your notifications in one place. Set reminders and make to-do lists. Jot down your thoughts and ideas with Notes. Share links, photos, and more right from the app you're in — post to Facebook, tweet, and send…