What is a Terminal Window?
A terminal window is a type of window, generally black with white text, where the output of terminal commands is displayed, and that may allow you to type terminal commands interpreted by a shell, in which case there will be a flashing text cursor at its last line.
An application whose sole purpose is to display a terminal window for interacting with the shell is called a terminal, e.g. Windows' Command Prompt and PowerShell, and KDE's Konsole are terminals.
Some applications may display a terminal window during start up that quickly disappears before displaying their graphical interfaces.
Running an application from a terminal window will display some text output on the window that wouldn't be visible otherwise. These are generally logs, warnings, and errors encountered by low-level libraries used by the application that aren't normally shown to the user.