Possible Duplicates:
What is a framework? What does it do? Why do we need a framework
What is the difference between a class library and a framework
Although I referred to various sources, I still can't understand the proper definition. What is meant by "application framework"?
Here's a simpler answer:
Application frameworks make writing applications easier.
Creating applications is hard. Applications have to provide input and output which they get through operating system semantics. Modern applications are usually GUI based and a GUI app is orders of magnitude more complex than a non-GUI app.
It's that simple. The framework takes all the complexities of interfacing with the operating system and simplifies them for you. It handles all the nitty-gritty details for you. Obviously certain frameworks do a better job at it than others.
There is one drawback to using an application framework that rarely seems to be discussed (presumably because we are all smiling about the amount of work we didn't have to do). In order to provide a simplified view of the operating environment, a framework has to box you into a certain 'style'. If your app is sufficiently different from the usual form of app, you are likely to end up frustrated in the framework as it will make doing what you want very difficult. This is partly because you now have to do all the things that the framework was hiding from you and partly because the framework is probably a closed system.