National debt. If all countries are in debt, who are they in debt to? Each other?
@Mourningmorningsleep - national debt is government borrowing but it doesn't generally involve borrowing from other countries. UK government needs to raise some cash to meet its current spending, so it issues debt (called gilts or government bonds), and mostly these are bought by investors, usually institutional investors such as pension funds and insurance companies.
So notionally, the UK Treasury gives the investor a piece of paper that says "I'll pay the owner of this piece of paper say £3 million pounds each year, for the next ten years and then pay you £100 million pounds at the end of the ten years". In return the investor will pay around £100 million up front to own that piece of paper. (The exact amount paid will be affected by the market's view of interest rates over the next 10 years).
So now the UK Treasury has around £100 million to use, and a commitment to repay interest (the £3million annual "coupons") and finally pay back £100 million at the end. Meanwhile the gilts (those pieces of paper) can be traded, so investors buy and sell, with the new owner being able to claim the remaining coupons and the final (maturity) payment. Because the market's view of interest rates changes all the time, the value of those pieces of paper fluctuates.
Companies can borrow in the same way by issuing corporate bonds. Their value fluctuates in the same way, but partly due to market's view of the credit-worthiness of the company (ie how likely they might default on repaying the debt).