CDOs, Synthetic CDOs and 2008 financial crisis

Question

I have recently been researching about the 2008 financial crisis and after hours of reading, I have been left with some gaps in my understanding of the events that lef to the collapse of the market in 2008. As far as I know, a synthetic CDO is an independent security and its value is derived from the premiums paid by investors and banks for credit default swaps on mortgage backed securities and other CDOs. What I cant fully understand is, how big was their role in the crisis, and if it was as big as i have read it was, who created them, and who were they sold to. Additionally, how is it exactly that Goldman Sachs fleeced AIG and what was their role in the crisis?