The tree package contains classes that allow you to build up visual trees, like the ones you are familiar with e.g. for browsing your file system. Expanding and collapsing tree nodes is handled automatically by showing or hiding the contained subtree structure.
A Tree contains items in an hierarchically structure. The first item inside a Tree is called the root. A tree always contains one single TreeFolder as the root widget which itself can contain several other items. A TreeFolder (which is also called node) can contain TreeFolder widgets or TreeFile widgets. The TreeFile widget (also called leaf) consists of an icon and a label.
Here are some links that demonstrate the usage of the widget: