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.
Microbiology therefore includes the study of both prokaryotic and eukaryotic microorganisms. In practise the majority of microbiology is concerned with bacteria and/or viruses although eukaryotic microbiology is also a very important branch of microbiology. Many diseases of animals (including humans) and plants are caused by bacteria, viruses, amoeba, and fungi. Bacteria are important in probiotics, they are used in food production (e.g. yoghurt and cheese) and biotechnology. Yeasts and fungi are important in food and drink production (e.g. wine, beer, bread) and are also used to produce important pharmaceuticals (e.g. antibiotics).