J.P. Morgan's blown trade, and its $2 billion bill, has everyone's attention. At its core was a bet on an abstruse corner of the credit market. Here's a quick guide to the credit indices that beached the London Whale.

The Wall Street Journal has reported that the trader, Bruno Iksil, had been placing bets on an index called "CDX.NA.IG.9"--the largest of a panoply of credit indices produced by data provider Markit.

The...