What is the difference between a stackpanel and a virtualizingstackpanel in WPF?
A VirtualizingStackPanel can offer performance benefits when working with very large collections. It does so by only rendering and processing a subset of the data which is visible to the user vs. processing the entire list of data. By creating only UI elements for the visible items, this can greatly reduce the amount of work it has to do.
This is really only handy though if
A StackPanel on the other hand, will up front create the controls for all elements contained within the StackPanel.
The VirtualizingStackPanel MSDN page has a decent discussion: http://msdn.microsoft.com/en-us/library/system.windows.controls.virtualizingstackpanel.aspx