Talk to Me Like I'm Stupid--Financial Derivatives

The rules are as I explained yesterday. I'm looking for the clearest possible explanation of financial derivatives, how they work in our world, and their place in the financial crisis. Do not think of this as a school essay assignment. Think of it as a conversation. This thread is the best model. Those of you who are ignorant like me can, and should, still participate. Please ask questions. I'm not looking at a single post in isolation; I'm looking at the whole thread. 

A final word of advice--attempting to show how smart you are will almost certainly cause you to lose. Understanding something is one skill. Explaining to people like they are stupid is another. For my own benefit, I'm hoping to cultivate the latter. I'll name a winner on Thursday.