Form 4868
Extends the filing deadline to 15 October. It does not extend the deadline to pay.
Application for Automatic Extension of Time To File U.S. Individual Income Tax Return. The extension is automatic — no reason is required and no approval is given — and runs to October 15. US persons abroad start from a different place: Regulation §1.6081-5 grants an automatic two-month extension to June 15 with nothing to file, provided you live and have your main place of business outside the US on the normal due date; you attach a statement to the eventual return explaining that you qualify. Form 4868 then extends from there to October. The distinction that costs money is that all of these extend time to file, not time to pay: interest accrues from April 15 on any unpaid balance regardless, and the late-payment penalty can apply from June. FBAR is not covered by any of this — it has its own automatic October extension.
Related
This glossary entry is general reference, not advice for your specific return. Start your filing on the residency step.