Great question (and a subject I love talking about).
Microbiology is the study of microbes.
What are microbes? Well microbes are really tiny organisms that are usually too small for us to see without the help of something like a microscope. The term ‘microbes’ covers quite a lot of things including:
-bacteria (like the bacteria I work on that can give us food poisoning)
-viruses (like the virus that gives us the common cold)
-algae (yes I know we can see some of these without a microscope – the term ‘microbe’ is a little confusing that way)
-fungi (yup we can see these too, e.g. we can see mushrooms – these are a type of microbe)
-protozoa (like the protozoa that causes malaria)
So you see microbiology covers the study of lots of different organisms.