US Citizen

Generally speaking, a US citizen is required to file a US tax return every year and report all their world-wide income and pay US tax on it, no matter where they physically live or perform the work.  If they also pay foreign income tax on the same income, the US will give them an offsetting deduction or credit.  If they live abroad more than 330 days, the taxpayer can claim a foreign earned income exclusion, meaning that income earned from working is excluded from US tax.  However, the taxpayer must still file a return to claim the exclusion, and any unearned income (interest, dividends, investments, gambling prizes, etc.) is still subject to US tax.