Microorganisms were divided into 6 distinct groups based on the phylogenic, morphological, and physiological characters. The major groups of microorganisms are

1. Bacteria are a phylogenetically related group of unicellular prokaryotic organisms distinct from Archaea

2. Archaea is a phylogenetically related group of unicellular prokaryotes which are primitive and distinct from bacteria

3. Fungi are a group of eukaryotic organisms lacking chlorophyll. They range in size and shape from single-celled yeast to multicellular mushrooms.

4. Algae refer to the group of eukaryotic organisms with chlorophyll. They range in size and shape from single-celled algae (Ex: Chlorella) to complex cellular structured plant-like algae (Ex. Kelp)

5. Protozoa are a group of eukaryotic organisms lacking cell walls. The morphology, nutrition, and physiology are different from other groups

6. Viruses are groups of non-cellular organisms, parasites, or pathogens to plant, animals, and other microorganisms. They are too small and cab be visualized only under electron microscopes

Microorganisms can be divided into two distinct groups based on the nucleus structure:

1. Prokaryotes

2. Eukaryotes

Prokaryotes

(pro-KAR-eye-ot-es)

(From Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -autos, pl. -toes; also spelled "procaryotes")

Prokaryotes are organisms without a cell nucleus (= karyon), or any other membrane-bound organelles. Most are unicellular, but some prokaryotes are multicellular.

The organism lacking true nucleus (membrane enclosed chromosome and nucleolus) and other organelles like mitochondria, Golgi body, endoplasmic reticulum etc. are referred as Prokaryotes. (Ex: Bacteria, Archaea)

Eukaryotes

Organisms whose cells are organized into complex structures by internal membranes and a cytoskeleton. The most characteristic membrane bound structure is the nucleus. This feature gives them their name, (also spelled "eucaryote,") which comes from the Greek ευ, meaning good/true, and κάρυον, meaning nut, referring to the nucleus. Animals, plants, fungi, and protists are eukaryotes.

The organism possessing membrane enclosed nucleus and other cell organelles are referred as Eukaryotes

Eukaryotic Cell versus Prokaryotic Cell comparison chart