Customs and Duties do not typically apply to tourists visiting with their own belongings.

Technically, the limit for goods brought back by Brazilians/residents to the country (via air travel) is US$500.00.  This would also apply to foreigners if they are planning to leave the goods in the country (say a gift for a Brazilian friend).

When passing through customs, there are two lines, Nothing to Declare and Goods to Declare. Those that go through the "Nothing to Declare" line can still be selected to pass their bags through X-Ray and/or have the bags physically searched.  In fact, travellers passing through this line are selected randomly for inspection by having to press a button. If the light turns green, then the traveller can pass without inspection. If the light turns red, the traveller has been randomly selected for inspection.

A note of warning for those who do not declare an item above the allowed value and pay the required duties. The penalty of being caught doing this is impound of the subject goods until a tax/fine is paid equal to the value of the good. This can be paid at the time, though.

Typically, however, this is not an issue unless the traveller is greatly and obviously exceeding the limit. Additionally, electronic equipment is more closely inspected as electronics in Brazil are much more expensive than say in the US or Asia.