Straight from the IRS:
Tax Treaty Tables
The United States has income tax treaties (or conventions) with a number of foreign countries under which residents (but not always citizens) of those countries are taxed at a reduced rate or are exempt from U.S. income taxes on certain income, profit or gain from sources within the United States.
Amounts subject to withholding tax under chapter 3 (generally fixed and determinable, annual or periodic income) may be exempt by reason of a treaty or subject to a reduced rate of tax.
These treaty tables provide a summary of many types of income that may be exempt or subject to a reduced rate of tax. For more details on the whether a tax treaty between the United States and a particular country offers a reduced rate of, or possibly a complete exemption from, U.S. income tax for residents of that particular country, refer to Publication 901, U.S. Tax Treaties. (Check this out! Click “Current Products” Publication 901)
For more details for withholding agents who pay income to foreign persons, including nonresident aliens, foreign corporations, foreign partnerships, foreign trusts, foreign estates, foreign governments, and international organizations, refer to Publication 515, Withholding of Tax on Nonresident Aliens and Foreign Entities.