Macworld — Apple (AAPL) engineers have been hard at work in the eight months since Steve Jobs first previewed Lion at last October's Back to the Mac event: After a brief recap on Lion’s new features at Monday's Worldwide Developers Conference keynote, Apple announced that the new OS would be shipping in July for just $30, and available solely from the Mac App Store.

Apple executives Phil Schiller and Craig Federighi were on-hand at the keynote to speak about and demonstrate ten of Lion’s more than 250 new features. To start, Schiller, Apple’s senior vice president of worldwide product marketing, gave an short overview of the first three—Multitouch gestures, fullscreen applications, and Mission Control—before turning it over to Federighi, vice president of OS X software, for a quick demonstration.

Federighi showcased several of the new gestures, including a two-finger swipe to move back and forth through Safari Webpages, before moving onto fullscreen applications. iPhoto, Safari, and Photo Booth all received quick demonstrations in fullscreen mode. In Lion, Photo Booth has gained several new effects: a new filter called Dizzy will animate a flock of birds circling over your head; you can also target specific facial features to enlarge and exaggerate them.

The Mac App Store—first introduced in Mac OS X 10.6.6—will also prominently feature in Lion. Schiller rattled off success stories from several prominent developers before moving on to talk about some of the new features coming to the store with Lion. Developers will be able to integrate in-app purchase, similar to applications within the iOS App Store, and built-in sandboxing; in addition, the store will offer push notifications and faster software updates, thanks to switching to an incremental update system.

Also demonstrated were Launchpad, the iOS-esque application-management tool; Resume, which saves the states of your windows and work on quit; Auto Save and Versions, which offer easy version management control in a Time Machine-like interface; AirDrop, a peer-to-peer Wi-Fi-based network; and an upgraded version of Mail, featuring a three-column interface, conversation view, search suggestions, and color-coding.

For developers, Lion will feature over 3000 new APIs they can incorporate into their applications, including versioning, push notifications, gesture tracking, fullscreen mode, and more.

Lion will be available in July for just $30, exclusively from the Mac App Store. A new developer preview will be available later in the day on Monday, as well.

We’ll update this story throughout the day with more information, so check back.