The Toolbox is the heart of the GIMP. It contains the most commonly used controls, and the highest-level menus. Two of the menus, File and Help, can also be accessed from the menubars located above each image display, although their contents are somewhat different. The Toolbox is the only place to access the Xtns (Extensions) menu.
You may find other entries in this menubar which are not described here. They are not part of standard GIMP, but have been added by some of the plug-ins. You can find more information about the additional menu entries in the documentation for the plug-ins.