Nice graphic showing the various sources the US borrows money from.

Here is a nice and concise graphic showing the who and how much we owe as a country to different entities.  A majority of our debt is owed to ourselves through government borrowing from Federal Trust Funds (i.e. Social Security), our own Federal Reserve Bank or from the domestic private sector. We do owe foreign entities about $5 Trillion, however.

Graphic from Planet Money