What is refactoring ? IDEA's feature list mentions 'refactoring'. What is it and where can I learn more about it ?

Eugene Belyaev

"Refactoring is the process of changing a software system in such a way that it does not alter the external behavior of the code yet improves its internal structure. It is a disciplined way to clean up code that minimizes the chances of introducing bugs. In essence when you refactor you are improving the design of the code after it has been written." Refactoring: improving the design of existing code / Martin Fowler (Addison-Wesley, 2000).