U.S. tax treaties or conventions with many foreign countries entitle U.S. residents to certain credits, deductions, exemptions, and reduced foreign tax rates. In this way, you may be able to pay less tax to those countries.
For example, most tax treaties allow U.S. residents to exempt part or all of their income for personal services from the treaty country's income tax if they are in the treaty country for a limited number of days.
Treaties also generally provide U.S. students, teachers, and trainees with special exemptions from the foreign treaty country's income tax. Publication 901 contains detailed information on tax treaties and tells you where you can get copies of them.