An alien is any person who is not a United States citizen. The Internal Revenue Service (IRS) classifies aliens as either resident Aliens or non-resident Aliens
Resident aliens generally are taxed on their worldwide income, similar to U.S. citizens.
To be classified as a resident alien, the individual must meet one of two tests:
1. Green Card Test
A non-resident alien is a lawful permanent resident of the U.S. at any time if they have been given the privilege, according to the immigration laws, of residing permanently as an immigrant. This status usually exists if the Bureau of Citizenship and Immigration Services has issued a green card.
2. Substantial Presence Test
A non-resident alien is classified as a resident alien for tax purposes if they were physically present in the U.S. for 31 days during the current year and 183 days during a three-year period that includes the current year and the two years immediately before that.
Information on the test is available at http://www.irs.gov/pub/irs-pdf/p519.pdf
If a person does not meet either the Green Card or Substantial Presence Test, then that person is classified as a non-resident alien.
Source: University of Washington Website and Duke University