I'm shopping for a used iphone to take to Brazil. Should I buy GSM or CDMA?
I'm shopping for a used iPhone to take to Brazil. Should I buy GSM or CDMA?

This answer applies to any country and (technically) any phone.  Therefore, I’m writing it as a canonical answer for future queries of this nature.  
You need to find out what type of technology is being used.
The best place would be to contact the local carrier.  In Brazil, it’s Vivo, Claro, TIM, and Oi.  You will want to check with them because different carriers use different technologies.  Here in the US, AT&amp;T uses GSM, while Verizon uses CDMA, for example.
However, you can get a good idea of what technologies are being used in a country with the help from the web site GSM Arena.  They maintain a fairly comprehensive database of the different radio types and frequencies used in a particular country.  Below is a screen grab of their entry for Brazil.

Here we can see that for 2G they use GSM, 3G they use UMTS and for 4G the use LTE.  CDMA is nowhere to be found.
It’s always best to check directly with your provider (whether it be cell services, email hosting, or even personal services) rather than with “the guy from the Internet.”  My information is accurate today, but I’m not a representative of the company and all of it can change tomorrow.  I’d rather you know how to obtain the correct info rather than hope the info is still correct.