Debt is an interesting tool.

In general debt is bad for you. Debt is mathematically a negative bond. If you own bonds, you should probably sell all of those before taking on debt. All of that said, there are sometimes good deals on debt and occasionally debt isn't a terrible deal.

The math behind debt is pretty easy, for $X of principal, you are charged Y% interest(per year) until all of $X(the principal) is paid off.

Debt is a good deal if you can almost risk-free make more than Y% interest on $X principal. That's not an easy thing to achieve, but it does happen on occasion.

Some situations where debt can make sense: