Registering Window Classes
Window "classes" in traditional programming for Windows define the characteristics of a "class" (not a C++ class) from which any number of windows can be created. This kind of class is a template or model for creating windows.
In a traditional program for Windows, without MFC, you process all messages to a window in its "window procedure" or "WndProc." A WndProc is associated with a window by means of a "window class registration" process. The main window is registered in the WinMain function, but other classes of windows can be registered anywhere in the application. Registration depends on a structure that contains a pointer to the WndProc function together with specifications for the cursor, background brush, and so forth. The structure is passed as a parameter, along with the string name of the class, in a prior call to the RegisterClass function. Thus, a registration class can be shared by multiple windows.
In contrast, most window class registration activity is done automatically in an MFC framework program. If you are using MFC, you typically derive a C++ window class from an existing library class using the normal C++ syntax for class inheritance. The framework still uses traditional "registration classes," and it provides several standard ones, registered for you when needed. You can register additional registration classes by calling the AfxRegisterWndClass global function and then passing the registered class to the Create member function of CWnd. As described here, the traditional "registration class" in Windows is not to be confused with a C++ class.
For more information, see Technical Note 1.