A "germ" is a more general term that refers to any microorganism that can cause disease. "Bacteria" is a particular type of germ. A "germ" might also be a virus or quite possibly some other thing. If I was a biologist, I could explain better but I am not.

A germ is a generic term and would cover bacteria, viruses, and other microorganisms. It's a non-medical and non-scientific term to mean anything that could potentially cause someone to become sick. Bacteria are single cell organisms. Not all bacteria are harmful and some are even used in probiotics, making cheese, or producing medicine. Another way of looking at it is that a bacterium (single form for bacteria) is a germ, but not all germs are bacteria.