Investment Banking - What do they do

An investment bank uses its proprietary book (own money) to lend others and invest. It started with the subprime crisis. Banks like Lehman, buy mortgage loans from other banks, and then package them to sell bonds against the loan pool. Often they add cash to make the loan pool more attractive, so that the bonds can be sold at a higher price.

Suppose mortgage was earning 6%, these bonds are sold at 4%. The difference is the spread which the investment bank earns. By selling these structured bonds, it raises money and frees capital. But when homebuyers started defaulting, these bonds lost their value. It all began like this, and then the virus spreads across markets.