Document extension and app types

extensions/docs/extension_and_app_types.md

+# Extension and App Types
+
+Generally, browser extensions cut across websites and web apps, while apps
+provide more isolated functionality.
+
+[TOC]
+
+![Summary of extension types showing browser extensions, packaged/platform apps,
+and hosted/bookmark apps](extension_types.png)
+
+## Browser extensions
+
+Browser extensions often provide an interactive toolbar icon, but can also run
+without any UI. They may interact with the browser or tab contents, and can
+request more extensive permissions than apps.
+
+## Apps
+
+### Platform app
+
+Platform apps (*v2 packaged apps*) are standalone applications that mostly run
+independently of the browser. Their windows look and feel like native
+applications but simply host the app's pages.
+
+Most apps, like Calculator and the Files app, create their window(s) and
+initialize a UI in response to Chrome's `chrome.app.runtime.onLaunched` event.
+Some apps don't show a window but work in the background instead. Platform apps
+can connect to more device types than browser extensions have access to.
+
+Platform apps are deprecated on non-Chrome OS platforms.
+
+### Packaged app (legacy)
+
+[Legacy (v1) packaged apps](https://developer.chrome.com/extensions/apps)
+combined the appearance of a [hosted app](#Hosted-app) -- a windowed wrapper
+around a website -- with the power of extension APIs. With the launch of
+platform apps and the app-specific APIs, legacy packaged apps are deprecated.
+
+### Hosted app
+
+A [hosted app](https://developer.chrome.com/webstore/hosted_apps) is mostly
+metadata: a web URL to launch, a list of associated URLs, and a list of HTML5
+permissions. Chrome ask for these permissions during the app's installation,
+allowing the associated URL to bypass the normal Chrome permission prompts for
+HTML5 features.
+
+### Bookmark app
+
+A bookmark app is a simplified hosted app that Chrome creates on demand. When
+the user taps "Add to desktop" (or "Add to shelf" on Chrome OS) in the Chrome
+menu, Chrome creates a barebones app whose manifest specifies the current tab's
+URL. A shortcut to this URL appears in chrome://apps using the site's favicon.
+
+Chrome then creates a desktop shortcut that will open a browser window with
+flags that specify the app and profile. Activating the icon launches the
+"bookmarked" URL in a tab or a window.
+
+## Manifest
+
+A particular manifest key in an extension's `manifest.json` file determines what
+kind of extension it is:
+
+* Platform app: `app.background.page` and/or `app.background.scripts`
+* Legacy packaged app: `app.launch.local_path`
+* Hosted app: `app.launch.web_url`
+* Browser extension: none of the above
+
+## Notes
+
+`Extension` is the class type for all extensions and apps, so technically
+speaking, an app *is-an* `Extension`. The word "extension" usually refers only
+to non-app extensions, a.k.a. *browser extensions*.
+
+## See also
+
+* [`Manifest::Type` declaration](https://cs.chromium.org/chromium/src/extensions/common/manifest.h?gs=cpp%253Aextensions%253A%253Aclass-Manifest%253A%253Aenum-Type%2540chromium%252F..%252F..%252Fextensions%252Fcommon%252Fmanifest.h%257Cdef&gsn=Type&ct=xref_usages)
+* Extensions (3rd-party developer documentation)
+    * [Extension APIs](https://developer.chrome.com/extensions/api_index)
+    * [Extension manifest file format](
+      https://developer.chrome.com/extensions/manifest)
+* Apps (3rd-party developer documentation)
+    * [Platform app APIs](https://developer.chrome.com/apps/api_index)
+    * [Platform app manifest file format](
+      https://developer.chrome.com/apps/manifest)
+    * [Choosing an app type](https://developer.chrome.com/webstore/choosing)
+    * Ancient article introducing the [motivation for apps (outdated)](
+      https://developer.chrome.com/webstore/apps_vs_extensions)