Refactoring is a disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior. Its heart is a series of small behavior preserving transformations. Each transformation (called a 'refactoring') does little, but a sequence of transformations can produce a significant restructuring. Since each refactoring is small, it's less likely to go wrong. The system is also kept fully working after each small refactoring, reducing the chances that a system can get seriously broken during the restructuring.
This is a simple list of refactorings both from the original book and some later sources. Refactorings marked with with have some extra material to what's in the Refactoring book. Refactorings marked with are in addition to those in the refactoring book. Refactorings with Link Only are references to refactorings described elsewhere.