According to the Free Online Dictionary, these terms are used in the US and Canada to describe the area of a city away from the centre (uptown), and the central or commercial area (downtown).
All the noise and the hurry
Seems to help, I know, downtown
Just listen to the music of the traffic in the city
Linger on the sidewalk where the neon signs are pretty
How can you lose?
The lights are much brighter there
You can forget all your troubles, forget all your cares and go
Downtown, things'll be great when you're
Downtown, no finer place for sure,
Downtown, everything's waiting for you
I think it's traditionally used mostly of New York. Most cities have a downtown, generally the cultural/commercial heart, probably the oldest part of town; but not many cities have an uptown, let alone a midtown like Manhattan.
Downtown Manhattan is south of 14th St. Midtown goes up to 59th St.