Individuals and resident aliens must file a tax return if their gross income exceeds certain thresholds based on their filing status and age. Resident aliens file similarly to US citizens using a Social Security Number, while nonresident aliens use form 1040NR and a Tax ID Number starting with 9. There are five basic parts to a tax return: personal information, income, deductions, credits, and the refund or amount due. Personal information includes the filer's name, Social Security Number, address, filing status, dependents, and exemptions.