The ListBox control displays a list of items. The user can select one or multiple items depending on the selection mode. The typical usage of a listbox in WPF is to bind its items to a list of business objects and display them by applying a data template.
How to define a Trigger for IsSelected in the DataTemplate
If you want to change the appearance of a ListBoxItem when it is selected, you have to bind the IsSelected property of the ListBoxItem. But this is a bit tricky, you have to use a relative source with FindAcestor to navigate up the visual tree until you reach the ListBoxItem.