A nonresident alien is a person from a foreign country working in the US who does not pass either the "green card" or "substantial presence" residency test, but is subject to federal income tax on US source income.
Generally, students (F-1 or J-1 visa status) will not meet the substantial presence test until they have been in the USA for 5 calendar years and visiting teachers/researchers (J-1 visa status) will not meet the substantial presence test until they have been in the USA for 2 calendar years.
Resident aliens are taxed the same as an U.S. citizen, while significant differences in the withholding, reporting and tax laws apply to nonresident aliens.
