Scene Tree Panel
Scene tree panel allows to browse the hierarchy of the scene, change it and do some other things. You may drag and drop items to reorder or re-parent them, there is a context menu that allows you to show and hide nodes, cut, copy, paste and delete them.
There is also a "Tools" menu there that allows you to do lighting, zooming, cloning and some other things.
How to group nodes
You may drag a node to another one to group them, so they move together. You may also create a "Group" node, using the "Scene > Create" menu. That special node type is not visible at rendering time and useful to put some other nodes together.