There many different living things that each of us may be familiar with. There are in fact so many different organisms on Earth that it is an overwhelming task to try to make general statements about them. To help deal with the great diversity of organisms, scientists have assigned them into general groups called Kingdoms. The members of each Kingdom share physical structural characteristics and similar feeding patterns. There are five Kingdoms in all. The Kingdom Monera contains microscopic organisms (bacteria and blue-green algae) that have their genetic material loose in a single cell. The cell thus has no compartments (like drawers in a refrigerator) where specific cell functions are carried out. The Kingdom Protista also includes one-celled organisms, but the amoebae and their relatives have compartmentalized cells. The genetic material that controls cell function and that passes on the traits of parents to their offspring is located in a compartment called the nucleus. Like the Kingdom Protista, the following three Kingdoms contain cells that have compartmentalized function. The organisms, however, are composed of many cells and are much larger and more complex. The Kingdom Fungi contains organisms (e.g., mushrooms and molds) that decompose organic matter into their simpler chemical compounds. When we talk about organic matter we are generally referring to organisms that have died or the non-living products produced by living organisms such as feces and shed skins. Decomposers absorb nutrients from the leftovers they have decomposed. The Kingdom Plantae contains organisms (e.g., trees and mosses) that make their own food using the energy from sunlight. Finally, the Kingdom Animalia contains all of the multi-celled organisms (like insects, fish and mammals) that depend on other living organisms for food.

The Kingdom Animalia has by far the greatest diversity of named organisms (approximately 1000,000 kinds or species) compared to the Plantae (300,000 species), the Fungi (70,000 species), the Protista (31,000 species) and the Monera (10,000 species). Unfortunately for many species, we only learn about them in time to witness their extinction. It is believed that one species becomes extinct every 20 minutes, which is between a hundred and a thousand times the “normal” extinction rate.

The animal kingdom is divided into about 30 phyla. The nine largest phyla contain the majority of species.

Indeed, one phyla, the arthropods, which includes insects and spiders, constitutes about 75 percent of all known animal species. More than 900,000 arthropods have been described, and according to some estimates there may be more than five million more.

The number of known species for all animals other than arthropods is about 250,000.

The largest group (formally "class") within the phyla of arthropods and the most diverse class in all kingdoms, is insects. Over 750,000 have been described. Some suppose that there are perhaps as many as three million different species of insects in the world.

The most diverse family of insect is beetles, with over 375,000 types identified.

Other large families of insects include butterflies and moths (more than 100,000 species), bees and wasps (more than 20,000 species), and ants (about 10,000 species).

With over 30,000 known species, spiders, which are not insects, constitute one of the large families of other kinds of arthropods.

Only two other phyla within the animal kingdom, the roundworms and the mollusks, are known to contain more than 100,000 species. All other phyla generally have far less.

The dominant phyla on the planet, the vertebrates, consists of less than 50,000 known species. The number of fish species is estimated to be more than 20,000, bird species number approximately 8,700, reptiles about 6,000, mammals about 4,500, and amphibians about 2,500.

Within mammals, the rodents are the most varied order, with 34 families and more than 1,700 species.