Marting Fowler Definition: Refactoring is a controlled technique for improving the design of an existing code base. Its essence is applying a series of small behavior-preserving transformations, each of which "too small to be worth doing". However the cumulative effect of each of these transformations is quite significant. By doing them in small steps you reduce the risk of introducing errors.
The main purpose of refactoring is to fight technical debt. It transforms a mess into clean code and simple design. Clean code:
Technical debt is a concept in programming that reflects the extra development work that arises when code that is easy to implement in the short run is used instead of applying the best overall solution.
Composing Methods Much of refactoring is devoted to correctly composing methods. In most cases, excessively long methods are the root of all evil. The vagaries of code inside these methods conceal the execution logic and make the method extremely hard to understand – and even harder to change The refactoring techniques in this group streamline methods, remove code duplication, and pave the way for future improvements.
Moving Features between Objects Even if you have distributed functionality among different classes in a less-than-perfect way, there is still hope. These refactoring techniques show how to safely move functionality between classes, create new classes, and hide implementation details from public access.
Organizing Data These refactoring techniques help with data handling, replacing primitives with rich class functionality. Another important result is untangling of class associations, which makes classes more portable and reusable.
Simplifying Conditional Expressions Conditionals tend to get more and more complicated in their logic over time, and there are yet more techniques to combat this as well.
Simplifying Method Calls These techniques make method calls simpler and easier to understand. This, in turn, simplifies the interfaces for interaction between classes.
Dealing with Generalization Abstraction has its own group of refactoring techniques, primarily associated with moving functionality along the class inheritance hierarchy, creating new classes and interfaces, and replacing inheritance with delegation and vice versa.