A collection is a set of Repository objects that are all connected to a common source object via a relationship collection.
A collection is displayed in the Browser as a node in the Browser tree. The objects within a particular object collection are displayed underneath the collection node, and connected to it. Using Windows Explorer as an analogy, the contents of a folder on your hard disk can be thought of as a collection of file and subfolder objects. In this case, the common source object to which all of the files and subfolders are connected is the parent folder object. In this analogy, only one type of collection is defined for a folder; namely, the collection of files and subfolders. Consequently, in Windows Explorer, the actual collection object is not displayed to you. In the Repository, however, multiple collections of different types can be attached to any single parent object. This is why the Repository Browser displays the collection between the parent object and the objects within the collection.
A relationship between objects is actually a relationship between the interfaces of two objects. This is a key point in working with Repository data: the Repository contains collections of relationships between interfaces of objects.