microbiologyMicrobiology studies the smallest life forms. They are only 1 to several cells in size. (Exception: the mushrooms, which are fungi, but with a decent volume.)
Micro - organisms have no tissues or organs.

They consist of organic substances. Which occur only in living organisms and their dead remains. Chemically, that are chains of two or more carbon atoms. At these C - atoms are others again. They are organic molecules: proteins, carbohydrates (e.g. sugar and starch) and fats.
Inorganic substances are from inanimate material. These are other compounds such as water, iron, copper, oxygen, iron salts, calcium salts, etc. Inorganic materials also come in many organisms.
Receiving and dispensing of substances is a metabolic process (metabolism) that can serve in a cell to build (build metabolism or assimilation) material and to draw energy from it (catabolism or dissimilation.)
A living organism has metabolism, may have offspring, and responds to environmental stimuli.
 
From large to small:       um (1 mm = 1000 microns)
unicellular algae                                1-500
protozoa                                            1-500
fungi                                                  2-300
yeasts                                               2-15
bacteria                                          0.3-10
archaea                                          0.1-15
viruses                                          0.01-0.3

These organisms are so small that we cannot see them. But we realize it is teeming everywhere. Without cultures, laboratory and devices we cannot investigate them. Bacteria and the large structures in their cells can still be seen with microscopes. Viruses are too small to be distinguished.
A microbe is a less used name for micro- organism that is to the naked eye (almost) not detectable. (tiny insects, bacteria, larvae...)