U.S. Time To Default

The USA defaults in

Days Hours Minutes Seconds

Unless Congress raises or cancels the Federal Debt Ceiling

What does this mean?
The U.S would default on its debt if the cash reserves at the Treasury is insufficient to pay interest and principal to U.S. bondholders. At the moment there is a federally legislated limit to federal debt, known as the debt ceiling. The U.S. government needs to keep borrowing as federal spending (social security, defence, etc) exceeds revenue (taxes), and so the U.S debt grows almost constantly.
What would happen?
If the U.S. defaulted on their debt than there would have to be immediate cuts to all government spending. In doing this the U.S. would certainly be plunged into recession and take the world with it. The global financial system would suffer hugely as 'riskless' collateral (assets locked up as a form of financial insurance) plunged in value and the world's reserve currency lost complete trust. In short this would not be good.
Has the U.S. ever defaulted before?
No, it has not. This is despite wars, crisises and political brinkmanship. If the U.S. defaulted today, it would be for purely political reasons.
Could this be avoided?
Yes, if Congress decides to raise the debt ceiling or scrap it. The only problem with this is that it requires Democrats and Republicans to work together as neither party controls Congress. The Republicans want concessions for raising the limit, notably spending cuts, while Democrats are keen to pass a 'clean' raise, in other words without concessions. At the moment it is a very high stakes game of chicken.
Is a default likely?
Not very, the U.S. government has raised the debt ceiling precariously close to defaulting before and both the Democrats and Republicans (probably) don't want to be responsible for tanking the world economy.
Won't this keep happening?
As long as the debt ceiling exists and the federal government is in deficit than the debt ceiling will have to keep rising, how often depends on how much they raise it by. It has long been argued that continually raising the federal debt limit is unsustainable because more debt means more interest on debt, which means more expenditure which means more debt is needed. These debate ceiling debates happen every few years, notably 2011 and 2013 in recent years.
Is the debt a burden for future generations?
It depends, they may just be able to keep raising the limit, as have current generations. If they do have to pay it off, though, then they will have to live with paying more tax and having lower federal spending proportionally.
Exactly how much debt does the U.S. have?
A lot, around $31.4 trillion or $31,400,000,000,000. This a crude approximation to the total value of every single house in the contiguous U.S.