The update has a number of improvements over 10.8 Mountain Lion, including a more robust notification system (which includes inline replies), better support for multiple monitors and full screen apps, a revamp of Finder, and the addition of iCloud Keychain for saving passwords.
This is the first time Apple has distributed a major update like Mavericks without charging — 10.8 Mountain Lion was priced at $19.99, while 10.7 Lion was $29.99. The free update is available to everyone running OS X 10.6 Snow Leopard and above.
NOTE: OS X Mavericks 10.9 download links is available for download, but Apple still updating its servers and pushing the download links on the Mac App Store. So if you didn't find the download links yet, please wait for a few hours until servers are back.