Collection or category is used to group similar pages/records together within an app, for example, we may have Page categories, Product collections, Blog categories etc. A collection is a page itself but its data also includes pages/records in that collection. How the data is displayed depends on each application (using app_collection.tpl template).

When saving a page, one or more collections can be specified for that page. They will be created if they do not exist before the page is added to those collections. This is the preferred way to create a new collection. 

A page will set the first collection entry as the main collection for that page (to be shown in the page's nagivation path) and list all other collections under the tab Settings. To remove the page from a collection, just click on the Remove button next to each collection.

Clicking on Collections in the App Listing menu will show all collections of different type. There you can edit each collection's content, clone or delete it.