I read that CDO's played a major role in the credit crisis of 2008. I would like to know what they are.
You can get better understanding if you go through the videos in below url
https://www.khanacademy.org/economics-finance-domain/core-finance/derivative-securities
Look for topics like MBS (Mortgage Backed Securities), CDO(Collateralised Debt Obligations) and CDS(credit Default Swaps)