Model organisms are well-studied, non-human species that are used in scientific research. The more scientists know about an organism's genetics, lifecycle, care and behavior, the better the species is for research. The research done with these organisms can be used to provide insight into our own biological processes. Organisms are generally small, often easy to care for, and relatively harmless.
Sometimes organisms are chosen based on their phylogeny.
Gene size is also an important consideration in selecting a model organism.