Barrett's oesophagus is :
Barrett's esophagus 

Is metaplastic change of distal esophageal mucosa from normal squamous epithelium to columnar epithelium, in response to chronic gastro-esophageal reflux.
The junction between squamous esophageal mucosa and gastric mucosa moves proximally
Three types of columnar epithelium have been described in Barrett's esophagus:


intestinal type - the most common type
the junctional type and
the gastric fundic type


Barrett's esophagus is a premalignant condition for adenocarcinoma esophagus
"The definition of Barrett's esophagus (BE) has evolved considerably over the past decade. Traditionally, BE was identified by the presence of columnar mucosa extending at least 3 cm into the esophagus. It is now recognized that the specialized intestinal type epithelium found in the Barrett's mucosa is the only tissue predisposed to malignant degeneration. Consequently, the diagnosis of BE is presently made given any length of endoscopically indentifiable columnar mucosa that proves on biopsy to show intestinal metaplasia." - Schwartz 9/e p831 (8/e, p868)

Also remember

The hallmark of intestinal metaplasia is the presence of intestinal goblet cells.
Lower oesophagus lined by columnar epithelium
Upper oesophagus lined by columnar epithelium
Lower oesophagus lined by ciliated epithelium
Lower oesophagus lined by pseudostratified epithelium