GUIPropView is a developer-oriented tool (a replacement for NirSoft's old WinLister) which displays detailed information on your system's open windows.
The upper pane of the program lists all open top-level windows (applications, probably). Details provided include handle, title, state (normal/ maximised/ minimised), visible, position, size, class name, style, process ID, thread ID, parent handle, process name, instance name, class module filename, and more.
Click one of these windows and its child windows are displayed in the lower pane, again with more details: handle, parent handle, ID, text, enabled, visible, Z-Order, position, size, class name, process ID, thread ID and more.
While most of this is strictly experts-only, GUIPropView does have one or two more general uses.
If a window is misbehaving - it's off the screen, or maybe maximised and won't go away - then right-clicking it in GUIPropView gives you options to minimise or maximise the window, centre it, set it to stay on top of everything else (or turn that behaviour off), hide the window, close it, or close the owning application.
And if you've got a mystery window on your desktop and you don't know who owns it, GUIProp may be able to tell you more. Just drag and drop the target icon from GUIPropView's toolbar to the window and check the results.