It looks like the tags drain and drainage are getting blurred. It seems like drain should refer to indoor plumbing issues and drainage should refer to outdoor/landscaping issues (or something along those lines), but I don't know enough about them to write good descriptions to draw that line. Anyone want to take a crack at it?
Drain: A channel or pipe carrying off surplus liquid, esp. rainwater or liquid waste.
Drainage: The means of removing surplus water or liquid waste; a system of drains.
A drain would refer to a single pipe, channel, or path used to remove liquid. Whereas drainage, refers to the entire system made up of one or many drains.
I would understand drainage to refer to the water flow through a landscape, or a field or subsoil, whereas a drain would be the man-made aspects - pipes, sewers, channels etc.