- ACTION_GET_CONTENT with MIME type */* and category CATEGORY_OPENABLE -- Display all pickers for data that can be opened with ContentResolver.openInputStream(), allowing the user to pick one of them and then some data inside of it and returning the resulting URI to the caller. This can be used, for example, in an e-mail application to allow the user to pick some data to include as an attachment.
There are a variety of standard Intent action and category constants defined in the Intent class, but applications can also define their own. These strings use Java-style scoping, to ensure they are unique -- for example, the standard ACTION_VIEW is called "android.intent.action.VIEW".
Put together, the set of actions, data types, categories, and extra data defines a language for the system allowing for the expression of phrases such as "call john smith's cell". As applications are added to the system, they can extend this language by adding new actions, types, and categories, or they can modify the behavior of existing phrases by supplying their own activities that handle them.