A visa is a document issued by a country giving a certain individual permission to formally request entrance to the country during a given period of time and for certain purposes. Visas are typically stamped or attached into the recipient’s passport or are sometimes issued as separate pieces of paper.
Generally speaking, US citizens may not need visas for countries in Latin America or Europe. Complete visa requirements can be viewed at www.traveldocs.com.
If a visa is required, it is your responsibility to obtain the specified visa determined by your trip leader.
NOTE: if the immigration officer at the airport asks you the purpose of your visit, say “tourism” unless directed otherwise by your field contact.