The word "virtualize" refers to a technique by which a subset of user interface (UI) elements are generated from a larger number of data items based on which items are visible on-screen. Generating many UI elements when only a few elements might be on the screen can adversely affect the performance of your application. The VirtualizingStackPanel calculates the number of visible items and works with the ItemContainerGenerator from an ItemsControl (such as ListBox or ListView) to create UI elements only for visible items. ...VirtualizingStackPanel is the default items host for the ListBox element. By default, the IsVirtualizing property is set to true.

[http://msdn.microsoft.com/en-us/library/system.windows.controls.virtualizingstackpanel.aspx]