Up until 1960, the Three Domain System of Biological Classification was the method used to classify all living organisms. It divided all organisms into the following groups: bacteria, archaea, and eukarya. The following paragraphs give some descriptions and characteristics of each group.
Bacteria: single-celled organisms that lack a cell nucleus. Their cell membranes are made of unbranched fatty acids and peptidoglycan, a compound of sugars and amino acids, forming the spongy ring around the cell. They succumb to most antibacterial antibiotics. They have their own distinct DNA, but it's usually in the form of simple ribosomes (rRNA). They reproduce rapidly by simple cell division. The three distinct shapes are rods, spheres, and spirals. They can live almost anywhere. An example of this group is a streptococcus bacterium.
Archaea: single-celled organisms that lack a cell nucleus. They have similar cell membranes to bacterium and behave similarly. They have actual genes to reproduce with and have the ability to use a variety of energy forms to make food. They reproduce by fission, fragmenting, or budding. Many species are found in the intestines of humans and animals, aiding their digestion process. Some are present in the oceans or in the soil and aid in the food chain. They can live in almost any environment. An example of this group is Nitrogen-fixing rizomes.
Eucarya: organisms that contain complex cellular systems. Each cell within a system has a nucleus, protoplasm, cytoplasm, advanced DNA strands (chromosomes), and structured cell membranes. Cells replicate themselves through mitosis or meiosis. This class of organisms is comprised of the animal, plant, fungi, and protist kingdoms. We are a classic example of the animal division of this group!
It is of interest to note that organisms are classified today through a five-kingdom system.