A National Debt happens when governments spend more than they actually have, and everyone getting the money agrees that's absolutely fine.

More Money Than There Is Money

The national debt is currently 13.7 trillion dollars (you can round up to 14 if you ignore that piddling little GDP of Denmark). The American people could work together, movie-montage style, collecting every single physical dollar and cent in existence and they'd still have to go to Vegas*, hit the roulette tables, lay it all on black and win. Four times in a row. That's how you know you're doomed - most people crushed by debt imagine cleaning up at the casino OR having all the money in the world. When you need both, you're buggered.

*Some of them would already be there

The Apocalypse Overdraft

Since the "amount due" now contains more zeros than an American Idol IQ report you might wonder how they get a half-trillion dollar extension every year. Unless they're expecting the gravitational pull of morbidly obese citizens to start compressing fat into diamond, surely the creditors must realize they aren't getting their money back?

They do, but they also realize that if they ever ask for it - even a bit of it - the very concept of money will collapse. Along with the rest of Western civilization. That's what happened during the Depression, the dot-com collapse, and the credit crunch - the money stopped having anything remotely to do with real wealth, and some selfish bugger asked to actually see these millions of dollars everyone was trading. All these events ended spectacularly, in the same way a car crashing into a petrol tanker is spectacular, and this time the debt isn't based on e-Bullshit or the stock market - it's anchored on the entire country. With "anchor" being a terrifyingly suitable metaphor to be tied to.

If they pretend they're going to get imaginary money in the future, the creditors can keep using their "investment" skills (as opposed to the "hunting and gathering skills" they'll need if they don't).

It's a pity Doctor Doom never thought of this. He keeps attacking banks with neutron bombs and having his ass handed to him. If he started small, maybe opened a checking account and took out an overdraft to buy some grenades, next thing you know he'd be in the back sharing cigars with the managers while the staff shovel the vault's contents into his heavily-mortgaged Armageddon Detonator. Then Spiderman's escorted off the premises for attacking a valued customer.

Debt Sales

Because we don't have a forest of magical money trees, nor a mountain made of gold, you might wonder where all this money is coming from. Simple - the ability to be owed money by America is being auctioned off almost every second day. It's been sold two or three times just this week. A quarter of this debt is now owned by foreign countries and a quarter of that goes to China. That means they get 3.125 states when the world repossesses, but if we're smart we can fob them off with Texas. It'll be awesome - they think all Americans are heavily armed inbreds, and it'll be true! They can set it up as a themepark! "U.S. Land", or more likely "Americ Park!" They'll be rolling around in their bulletproof buses, watching tremors in glasses of water as the tour attractions lumber past.

"And they all have guns? Even the ones who failed basic educational tests?"
"Especially them! In fact they have the most guns of all!"
And our Asian visitors will be scared, and go home even more determined to "actually work" and "not try to make a living by suing their employers, governments and people they see in the street."

Of course, in true action-movie style it'll all go wrong and heavily armed Americans will rampage and start invading other countries, and .... wait.

Owing Everything

The national debt is expected to hit 100% of the Gross Domestic Product around 2019, at which point we can only presume America will be repossessed by everyone who bought the debt sales above. Since 2.2 trillion is owned by the Federal Old Age and Survivors Trust Fund, this could well mean pensioners declaring your city to be their lawn and demanding you get off it. But since public services will have broken down by then you can easily hurl a razor-sharp boomerang through their decrepit necks to protect your precious water.

(Note: start learning how to sharpen boomerangs by 2015 at the very latest, it takes a while to get the knack.)