Copied from the help file
Ghost towns are abandoned towns. When the last member of a town leaves it becomes a ghost town.
Buildings in ghost towns cannot be used anymore. The hotel and bank close down and so do the merchants.
It is possible to reawaken a ghost town. Doing this costs just as much as founding a new town. The big advantage of taking over a ghost town is that the buildings keep their old levels. When taking over a town you are shown the points that the town will have after completion.