A window is a graphical interface element used to display the contents of an application for the user to view and interact with. A window is usually a rectangular area that can be resized and is generally editable according to the capabilities and limitations imposed on it by the application providing it. The window is essential in facilitating multitasking in a modern operating system, as it allows users to visually and manually switch between running applications and make general interactions with the operating system.
Techopedia explains Window
The concept of the window was first developed at the Stanford Research Institute by Douglas Engelbert. Their earliest prototypes had multiple windows, but there was no way of distinguishing between them because they had no borders, title bars or the other GUI elements we know today. The research was continued at Xerox Palo Alto Research Center (PARC) led by Alan Kay, which later in 1980s came up with the term WIMP, meaning "window, icon, menu and pointer." Apple developed an interface based on WIMP and used it in their Lisa computers. A little later Microsoft released its own OS with a windowing system known simply as Microsoft Windows.
There are many types and elements of windows, but the most common is the application or main window, which is used to directly interface the user and the application. It contains the border frame, title bar and some control buttons like the minimize, maximize and close. Contrary to general knowledge, there are many types of windows and a number of UI elements such as buttons and edit boxes are themselves windows. They are called control windows and are placed relative to the application window and move along with it, as well as communicate with the application window by giving it click notifications, for example.