In many cases the data that you work with is a collection of objects. For example, a common scenario in data binding is to use an ItemsControl such as a ListBox, ListView, or TreeView to display a collection of records.

You can enumerate over any collection that implements the IEnumerable interface. However, to set up dynamic bindings so that insertions or deletions in the collection update the UI automatically, the collection must implement the INotifyCollectionChanged interface. This interface exposes the CollectionChanged event, an event that should be raised whenever the underlying collection changes.

WPF provides the ObservableCollection<T> class, which is a built-in implementation of a data collection that implements the INotifyCollectionChanged interface.

[http://msdn.microsoft.com/en-us/library/ms668604.aspx]