Code refactoring is a "disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior"

Could anyone explain this definition? I find it kind of blurry.

Which is the best technique to do it? Are these the only ones?

Which are the obstacles one would meet when refactoring code?

What to take into consideration when switching between IDEs?

Do tools which help refactoring exist? If yes, which one would you recommend using?

Any references to links where I can find out more on this subject or to books that have been written are welcomed.

The questions above are for any programming language, but specific examples are appreciated. Anything that would clarify this for me is.