What is UIVirtualization in WPF

wrt ... Silverlight or WPF it is about simulated Display of items.
For Ex. when you have millions of rows bound to Listbox(an Items template) ....
We dont want UI to put efforts on NON visible part of items in Listbox(These items will be visible when user scrolls up or down the scrollbar.)

Now the scrolling contailer is responsible of this virtualization.

Ex:
<ListBox VirtualizingStackPanel.IsVirtualizing="True"
...
ItemsSource="......." Height=
...
/>