Let's use Visual Studio as an example.  To start, Visual Studio is made up of "regions" or areas within the application; it has a menu bar (or in .NET 2.0 Windows control terms the MenuStrip).  Below it is multiple toolbars (or ToolStrips) that contains any toolbars buttons, drop downs, etc., which can be turned on or off through a context menu (or ContextMenuStrip) so that only certain toolbars are visible when needed.  The Server Explorer, Toolbox, Solution Explorer, and Team Explorer windows are called tool windows or sidebars.  These windows can appear in the left, right or bottom sides of the window, and can either be fixed and resizable.  Windows can also be collapsible and pinnable, for space reasons.  There are several toolkits available, such as Divelement's SanDock.  That leaves us with the documents that are tabbed in the center, and the status bar on the bottom, all which completes the major user interface items in an application.

Countless vendors use the Office API to create a user interface or component for the variety of Office products available; the same capabilities are being leveraged in Visual Studio, as companies realize that they can embed their software directly into the tool instead of creating a new interface.  The real benefit is that rather than having to develop your own custom tool, you can learn how to create an add-on and use it within the application.  There is less development effort in this approach.