Microorganisms are tiny living things that are not visible to the naked human eye as individuals, only as clusters. These microorganisms may be simple single-celled organisms, or they may have multiple cells with distinct growth and behavior patterns. The function of microorganisms varies widely according to the type, and the definition varies somewhat according to the source. However, there are several major groups of microorganisms that help make identification and classification easier.
Nearly all algae is considered a simple plant, though not all of them are microorganisms. There are hundreds of different types of algae, and among them are a number of unicellular and micro-scale multicellular types. These types of algae clump together by the millions and thereby become readily visible to the human eye.
All bacteria are considered microorganisms. These infinitesimal organisms live in practically everything and range from the exceptionally beneficial to the potentially deadly. Many types of bacteria need some type of host in order to survive, and they adapt to very specialized environments such as inside an animal's gut or at the bottom of the deepest part of the ocean.
Not all fungus is considered a microorganism, but there are a few that are used every day in normal modern life. Yeast is a microorganism that is used in most types of bread. Molds not only appear in damp or neglected areas where they're not welcome, but they're also used for medicines and other beneficial purposes.
Protists or protozoans also include some types of what's termed as single-celled "slime" organisms. These appear very similar to algae, but are more animalistic in their behaviors and energy acquisition methods. They live in both soil and water, depending on the type, and generally range around 10-50 micrometers in size. However, very high pressures allow protozoans to grow to amazing sizes, allowing deep-sea single-celled organisms that measure up to about eight inches in diameter. They range from dangerous parasites to herbivorous creatures that feed on microscopic fungi and algae.
For lack of a better descriptor that covers them all, a major grouping of microorganisms is such things as rotifers, planarians and helminthes. This last includes many types of parasitic worms that afflict mammals such as humans. Planarians generally live in water and feed off of all kinds of dead matter. Rotifers serve as a food source for many aquatic creatures and also aid in the decomposition of matter in soil.
While there is some disagreement about whether or not a virus is a living thing, many consider it another microorganism. Certainly, viruses do behave in a way that resembles a living thing, complete with reproductive behaviors and survival mechanisms. In addition, because viruses are made up of organic material and can be killed, they are frequently listed as being alive. Viruses are responsible for a wide range of diseases in humans and animals and reproduce in such a way that makes them extremely difficult to kill.
The major groups of microorganisms will change somewhat according to the source. It is important to note, however, that there are literally millions of different types of microorganisms that facilitate functions that are often taken for granted.