Messages form the heart of Windows programming. Whenever anything happens on a
Windows machine, such as a user clicking the mouse or pressing a key, a message is
triggered and sent to one or more windows, which do something about it. Visual C++
makes it easy for you to write code that catches these messages and acts on them.
Chapter 3, "Messages and Commands," explains the concept of messages and how MFC and
other aspects of Visual C++ enable you to deal with them.