National Debt reaches $8,000,000,000,000 (8 trillion dollars)

[clambunny]

the world bank last year said the single greatest threat to economic security in the world was US's national debt. anyone want to explain what it is to me? i gather it is our exports minus our imports, but when we import something like if i just bought a kimono from japan online wouldn't i immeadiately have to pay for that product, just like all things we import? so how could a debt accumulate?