Southern exposure simply means that a wall of a given structure faces south. Houses with southern exposure (in the Northern hemisphere) will get more sun in the winter. As the Earth passes through the seasons, the summer sun slowly drops from directly (well, nearly) overhead at mid-day to a winter sun that is distinctly nearer the southern horizon.
Many buildings take advantage of this by having windows and maybe even solar collectors on the south side of the house, collecting as much as possible of the sun's light throughout the year. Many times the summer sun is minimized by building with overhanging eaves, blocking out the hot summer sun from overhead, but still allowing the lower winter sun to shine in.
All this would be reversed in the Southern hemisphere, where I assume that people seek houses with Northern exposure.
StrawberryFrog says North-facing is indeed good for sun in the Southern Hemisphere. The thing is, while there is lots of land up near the north pole, there's not so much near the south pole. The bottom end of Africa has a Mediterranean climate like Spain or Northern California, so facing the sun is not so vital.
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