Each window class is associated with a window procedure, which defines and/or establishes access to data objects and performs processing related to that window class. In object-oriented terms, the window procedure contains the methods to carry out actions upon the data object referenced by the window.
A window procedure is normally invoked by Presentation Manager on behalf of the application. The window procedure interprets messages passed by Presentation Manager to the window and invokes a method (that is, a coherent set of application statements and/or routines) depending on the nature and contents of the message.
See Window Procedures for a more complete discussion of window procedures' structure and behavior.