• Discover Mac apps: Browse featured apps, top charts, and categories, or search for something specific. Read detailed app descriptions and user reviews, and flip through screen shots.
• Buy and install: Easily purchase apps with your iTunes account. Apps install in one step and are quickly available from the Dock.
• App updates: The Mac App Store keeps track of all your purchased apps and notifies you when free updates are available.
To learn more about the Mac App Store, visit: http://www.apple.com/mac/app-store.
For detailed information on this update, please visit this website: http://support.apple.com/kb/HT4459.
For information on the security content of this update, please visit: http://support.apple.com/kb/HT1222.
UPDATE: 10:09am ET: Apple now has the standalone installers available here.