São Paulo is the largest city in Brazil, with a city population of about 11 million and almost 20 million in its metropolitan region.
It is the capital of the Southeastern state of São Paulo, and also a beehive of activity that offers a jovial nightlife and an intense cultural experience. São Paulo is one of the richest cities in the southern hemisphere, though inequality between the classes typically observed in Brazil is blatant. Historically attractive to immigrants as well as (somewhat later) Brazilians from other states, it's one of the most diverse cities in the world.
São Paulo, or Sampa as it is also often called, is also probably one of the most underrated cities tourism-wise, often shaded by other places in the Brazilian sun & beach circuit such as Rio de Janeiro and Salvador. It is in fact a great city to explore, with its own idiosyncrasies, the exquisite way of living of its inhabitants, not to mention the world-class restaurants and diverse regional and international cuisine available to all tastes. If there is a major attraction to this city, it is the excellent quality of its restaurants and the variety of cultural activities on display.
A large sprawling city can present numerous challenges to sensibilities. São Paulo is no exception. Although the first impression might be that of a grey concrete jungle, soon it becomes apparent that the city has a great number of pockets of beauty. The population and environment of São Paulo is diverse, and districts within it range from extremely luxurious areas to hovels housing the poor and destitute, located usually in suburbia far from the so-called "expanded center".
São Paulo, together with Rio de Janeiro, is the spot where most visitors from abroad land in Brazil. While a complete experience of the city would take a few weeks (since the lifestyle of Paulistanos and every-day routine in the city are huge attractions in themselves), it's possible to visit all major sites within three days.
Staying a little longer than that is always a nice idea. As the financial and cultural center of the country, the city is a sea of possibilities.