Simply put, the FBAR is a reporting form for those who hold $10,000 or more in a foreign financial account. $10,000 refers to aggregate balances of foreign accounts.
When is the FBAR filing deadline?
The FBAR is due when your tax return is due. This is not a typo, it used to be due on June 30th but that changed as of the 2016 filing season. Also new - if you miss the deadline, you get an automatic 6 month extension; you do not need to submit any forms for an extension. So if you missed filing your 2016 FBAR you actually aren't late (yet).
What should I do if I have late or unfiled FBAR Forms? Or if I am under audit or investigiation?