Windows Within Windows
As a rule, differences between programs manifest themselves as differences between windows in four different ways:
One important difference between workspaces concerns the structure of the workspace. Many programs, like the Notepad Text Editor and the Paintbrush drawing program, have windows whose workspaces contain a single, monolithic working area.
Other programs--like Program Manager, File Manager , Word, Access, Excel, and PowerPoint -- have windows that, in turn, contain windows. The picture below is an example:

As a rule, a "window that contains windows" represents a program that allows you to shift back and forth between different tasks. In Program Manager, for example, each inner window represents a different program. In Word, each inner window represents a different open document. And, in File Manager, each inner window represents a different part of the file system.