Starting from a simple loan (remember them), credit markets have evolved many innovations to cater to an increasing need for leverage (intrinsic firm leverage - levered loans to high-yield bond market; and extrinsic instrument leverage - securitizations and derivatives) for issuers, traders, and investors. However, as the following maze of the history of debt, NY Times shows these have led to many costly crises (and will do in the future...)

Source: NY Times