GSM stands for Global System for Mobile communication and is the most widely used mobile communication system in the world. GSM phones work by sending information digitally through a frequency range or bandwidth. It is estimated that GSM has between 1 to 3 billion users worldwide and is available in 190 countries. The wide-spread use of GSM phones and technology throughout the world has made traveling with GSM phones much easier. GSM network operators have roaming agreements with foreign operators and so users can often continue to use their mobile phones when they travel to other countries.
The different range of GSM frequencies, however, means that certain phones might not work in certain countries. GSM phones that are only a single "band" will only be compatible in countries which use the same major bandwidth. For instance, GSM-900 and GSM-1800 are used in many parts of the world including Europe, Africa and Oceania. GSM-850 and GSM-1900 are used in the United States, Canada, and many other countries in the Americas. You can avoid compatibility problems by getting a tri-band or quad-band phone, which supports more than one frequency and, in the case of quad-band, almost all of them.
Bolivia, in particular, uses GSM 1900 and GSM 850. The networks that carry these frequencies are Entel, Neuvatel PCS and Telefonica Cellular de Bolivia. If you are coming from the United State, Canada and many other parts of the Americas, your phone should work in Bolivia. It is worthwhile to check your phone's compatibility with your cell phone provider, however, before you leave the country.