Check out the IRS website.
Summary available at -
http://en.wikipedia.org/wiki/IRS_tax_forms
You can apply for exemption from paying foreign earned income if you are a non US person, you just need to fill the relevant form to declare the income and certify that you are exempt.
US income tax laws are complex and if your earnings are substantial get a certified Chartered Account, my firm uses PWC for tax consulting.
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Some extracts as below
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W-8 Series
The Form W-8BEN, Certificate of Foreign Status of Beneficial Owner for United States Tax Withholding, is used by foreign persons (including corporations) to certify their non-U.S. status.
The form establishes that one is a non-resident alien or foreign corporation, to avoid or reduce tax withholding from U.S. source income, such as rents from U.S.
property, interest on U.S. bank deposits or dividends paid by U.S. corporations.
The form is not used for U.S. wages and salaries earned by non-resident aliens (in which case Form W-4 is used), or for U.S. freelance (dependent personal services) income (in which case Form 8233 is used).
The form requires the foreign person to provide a U.S. Taxpayer Identification Number unless the U.S. income is dividends or interest from actively traded or similar investments. Other W-forms handle other international issues.
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