An example of good debt is the debt on the apartment houses I own. That debt is good only as long as there are tenants to pay my mortgages. If tenants stop paying their rent, my good debt turns into bad debt.
I began playing Monopoly for real when I was 26 years old. Today, my wife and I have approximately 1,400 little green houses - each paying us monthly. You do not have to be a rocket scientist or have a Harvard degree to play Monopoly for real.
Bad debt is debt that makes you poorer. I count the mortgage on my home as bad debt, because I'm the one paying on it. Other forms of bad debt are car payments, credit card balances, or other consumer loans.