Card trees allow you to define a hierarchy of parent-child relationships between any card types. A card type can participate in any number of trees, and will inherit relationship properties for each tree it is a member of allowing cards to be linked to their parents in the tree. These relationships can be viewed graphically, as a tree view or hierarchy view, and cards can easily be added and moved around in the tree view. You can also see the members of a tree in the list and grid views. Aggregate properties can be defined for each card tree. For example you could create an aggregate property to sum the estimates for all the story cards assigned to an iteration card and show that aggregate property in the iteration card.
You can use a card tree as a filter condition when viewing cards in a grid or list view by selecting the card tree in the Select tree control located in the header of any list of cards. This will restrict the cards listed to only those which are currently included in the specified tree. This restriction will be applied in addition to any other filter conditions you might have set.