Barrett's oesophagus is a condition which occurs in approximately 10% of patients who suffer with long-term acid reflux.
It is thought to be a protective mechanism against the pain of the reflux. It is not clear why some people develop it and many others do not.
Click here to look at a very informative American website about Barrett's Oesophagus. Some of the information seems to be slightly out of date now, but it gives an excellent overview of the condition and treatment. You can download the entire website to browse later if you want by clicking here.