Packaged apps have access to Chrome APIs and services not available to traditional web sites. You can build powerful apps that interact with network and hardware devices, media tools, and much more. Here's a short list of examples:
Watch the Chrome Apps Google I/O presentation for an in-depth introduction.
When a user opens a packaged app, their focus is specifically on the tasks relating to the app. Packaged apps have no traditional chrome: the omnibox (address bar), tab strip, and other browser interface elements no longer appear. Like native apps, they don’t live within the browser. When launched, packaged apps can open in windows that look like this (and you can style your windows in all different ways):
Packaged app pages always load locally. This allows apps to be less dependent on the network. Once a user installs an app, they have full control over the app's lifecycle. Apps open and close quickly, and the system can shut apps down at any time to improve performance. Users can fully uninstall apps.
Without any effort on your part, your apps will launch offline. But you will need to put some effort into making sure user data is stored locally while offline and then synced back up to your data server once online (see Offline First).
Packaged apps are modified web apps. You use the same code, frameworks, and tools of the web platform to write your apps. Some browser features have been removed, other web APIs have been disabled or changed to improve security and programming practices.
New features have been added to help you build more native-like apps. The app container and programming models control how packaged apps look and behave. These models aim to provide users with a more native experience. Powerful APIs have been added so your apps can have native-like capabilities, and a serious security model is enforced to make sure these APIs are not abused.
To learn more about how to develop packaged apps: