(from Wikipedia entry)

In computer programming, an application framework is a term usually used to refer to a set of libraries or classes that are used to implement the standard structure of an application for a specific operating system. By bundling a large amount of reusable code into a framework, much time is saved for the developer, since he/she is saved the task of rewriting large amounts of standard code for each new application that is developed. Application frameworks became popular with the rise of the graphical user interface (GUI), since these tended to promote a standard structure for applications. It is also much simpler to create automatic GUI creation tools when a standard framework is used, since the underlying code structure of the application is known in advance. Object-oriented programming techniques are usually used to implement frameworks such that the unique parts of an application can simply inherit from pre-existing classes in the framework.

There are also a number of frameworks which will create identical applications for Linux, Macintosh, and Windows from the same source code, such as the widget toolkits wxWidgets or FOX toolkit.