Buenos Aires is a city with over 13 million people, a huge city by any standards. Neighborhoods are, as any other part of the world, arranged by socio-economic class, so once you known where you go, you can have an idea of the kind of people you will find. Downtown is the only exception because people do not live there. All folks are there for working reasons and you can see the president of a bank and a beggar sharing the same spot. This social contrast makes interesting to take a look at the downtown.
Other parts of the city have interest for other reasons and you will find a more detailed description in the specific neighborhood sections.