The BBC has a smart graphic to illustrate:
the gross external, or foreign, debt of some of the main players in the eurozone as well as other big world economies. The arrows show how much money is owed by each country to banks in other nations.
Oh yes, please be seated when checking the US's role.

Link:http://www.bbc.co.uk/news/business-15748696

Finally to those who decry simplicity yes there are gaps, such as how much is owed to PRC.