For tax purposes, an alien is an individual who is not a U.S. citizen. Aliens are classified as nonresident aliens and resident aliens. Publication 519 will help you determine your status and give you information you will need to file your U.S. tax return.
Other Items You May Find Useful
About Publication 54, Tax Guide for U.S. Citizens and Resident Aliens Abroad
About Publication 463, Travel, Entertainment, Gift, and Car Expenses
About Publication 501, Exemptions, Standard Deduction, and Filing Information