Microbiology is the study of small living things. Generally this means living things that are too small to see without the use of a microscope. These life forms are called microorganisms or microbes. Microorganisms include bacteria, archaea (a type of prokaryote a bit like bacteria but they have a distinct evolutionary origin), viruses, protozoa (single-cell eukaryotes like amoeba), microscopic fungi and yeasts, and microscopic algae (plant-like organisms).

Microorganisms were discovered over three hundred years ago and it is thought that many new microbes have yet to be discovered. Microbiology is a wide area of science that includes bacteriology, virology, mycology, phycology, parasitology, and other branches of biology.Most living things can be classified into prokaryotes or eukaryotes depending on whether their nuclear material (for example DNA) is surrounded by a membrane or not. Archaea and bacteria are prokaryotes. Most animals (including humans) and plants are eukaryotes. Protozoa, fungi, yeasts, and algae are eukaryotes. Viruses are a little different. Traditional classification systems do not classify viruses as living organisms. However in practise they are considered microorganisms. The study of viruses is called virology.