Who is a "nonresident alien for tax purposes"?

Who is a "nonresident alien for tax purposes"?

An individual who has not met or passed the substantial presence test by virtue of the number of days physically present in the U.S. or has not been granted lawful permanent residence in the U.S. as a permanent resident alien. A nonresident alien is taxed only on his/her income from U.S. sources, using special tax withholding, reporting, and filing requirements different from those of a U.S. citizen or resident aliens for tax purposes.