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Windows Messages
Windows passes input to a window procedure in the form of messages. Messages are generated by Windows and by applications. Windows generates a
message at each input event  for example, when the user types, moves the mouse, or clicks a control such
as a scroll bar. Windows also generates messages in response to changes in the
system brought about by an application, such as when an application changes the
pool of system font resources or resizes one of its windows. An application can
generate messages to direct its own windows to perform tasks or to communicate
with windows in other applications.
Windows sends a message to a window procedure with a set of four parameters: a
window handle, a message identifier, and two 32-bit values called message parameters. The window handle identifies the window for which the message is intended. Windows uses it to
determine which window procedure to send the message to.
A message identifier is a named constant that identifies the purpose of a message. When a window
procedure receives a message, it uses a message identifier to determine how to
process the message. For example, the message identifier WM_PAINT tells the window procedure that the window's client area has changed and must
be repainted.
Message parameters specify data or the location of data used by a window
procedure when processing a message. The meaning and value of the message parameters
depend on the message. A message parameter can contain an integer, packed bit
flags, a pointer to a structure containing additional data, and so on. When a
message does not use message parameters, they are typically set to NULL. A
window procedure must check the message identifier to determine how to interpret the
message parameters.
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