Americans are required to state global income over some threshold (I believe it was around $91K this tax year). Few other countries apply a tax to wages earned overseas by citizens. After passing a minimum threshold, tax treaties are applied to determine precise tax liability for the year.
"If you are a U.S. citizen or resident alien, the rules for filing income, estate, and gift tax returns and paying estimated tax are generally the same whether you are in the United States or abroad. Your worldwide income is subject to U.S. income tax, regardless of where you reside."
"If you are a U.S. citizen or resident alien, you must report income from all sources within and outside of the U.S. This is true whether or not you receive a Form W-2 Wage and Tax Statement, a Form 1099 (Information Return) or the foreign equivalents."