In science, a model organism is an organism often used for study and experiments, and may be considered a representative example of a class of organisms, or have a particularly interesting or easily-studied physiology or psychology. Model organisms are also selected for convenience and fast reproduction. Examples include the bacterium Escherichia coli, the nematode Caenorhabditis elegans, fruit flies (Drosophila melanogaster), albino brown rats (Rattus norvegicus, also called "lab rats"), mice (Mus musculus), and humans (Homo sapiens). This is only a short list — in total, there are many dozens of model organisms, from viruses to fungi to plants and both invertebrates and vertebrates.
Model organisms have taught scientists, and by extension, the human race, tremendous amounts about biology and psychology. Because of model organisms, we know brains are made of neurons, bodies work in a purely mechanical fashion (rather than being animated by a "vital force," as argued by Henri Bergson in 1907), how each organ works in great detail, and how diseases and mutations effect the body.
To the dismay of animal rights activists, model organisms are sometimes thoroughly abused or tortured by violent experimentation. Many institutions have been responsive by experimenting on lower model organisms or using computer simulations when possible, conducting more humane experiments, and refraining from experiments on higher model organisms unless absolutely required.
The complexity of the model organism at hand is usually a clue to what it is being used to study. For instance, nematodes are quite simple multicellular organisms, and are often used to study the basics of physiology, such as the way the brain directs body parts. The superlative example of this is the nematode C. elegans, which has been studied so extensively, scientists know where every cell in its body goes, and how these cells develop and differentiate during the process of embryogenesis. More complex animals, such as cats or rats, are used for more complex studies, such as studies of vision or subtle psychological features.