If you miss the April 15 due date (established by the IRS for filing and paying any taxes due to the federal government), the world isn't going to end, but you might be charged some late fees.
The IRS charges two different kinds of late fees: failure-to-file and failure-to-pay penalties. The failure-to-file penalty is much higher than the failure-to-pay penalty — 5 percent of unpaid taxes per month versus 0.5 of 1 percent per month — so the best advice is to file on time even if you can't pay your full tax bill [source: IRS].
Yes, the IRS lets everyone apply for an automatic six-month filing extension, but read the fine print. An extension gives you six more months to prepare and file your tax return, but it does not excuse you from paying your taxes on time. When you file for the extension, you are asked to pay an estimation of your outstanding tax bill. If you don't cover at least 90 percent of your taxes due by April 15, the failure-to-pay penalty still applies.