Every window is a member of a window class. An application must specify a window class when it creates a window. Each window class has an associated window procedure that is used by all windows of the same class. The window procedure handles messages for all windows of that class and, therefore, controls the behavior and appearance of the window.

A window class must be registered before an application can create a window of that class. Registering a window class associates a window procedure and class styles with a class name. When an application specifies the class name in a window-creation function such as WinCreateWindow, the system creates a window that uses the window procedure and styles associated with the class name.

An application can register private classes or use preregistered public window classes.