If you stop early enough, a lot of the physical effects of smoking i.e. getting out of breath easily, coughing etc can be improved, but if someone was to examine your lungs they would be able to tell that you have been a smoker. The lungs do have the ability to clean themselves, but they do not "regenerate" to pre-smoking state.
If you were to be diagnosed with something like COPD through smoking, that would not go away if you were to stop smoking. However, it would not get any worse if you stopped smoking.
In short, smoking is a fucking stupid thing to do and will f**k your lungs right up. Don't do it.