Model organinsm

A model organism is a well established experimental biological system. Common model organisms include: yeast (eg. Saccharomyces cerevisiae and Schizosaccharomyces pombe); some plants (eg. Arabidopsis thaliana); flys (eg. Drosophila); worms (eg. C. elegans); fish (eg. zebra fish) and mice (eg. Mus musculus).

There are many characteristics which an organism must suit in order for it to be an effective model organism. Some of these characteristics include; 

Rapid rate of development (to maturity)
Easily manipulated (genetically)
Short life span (life cycle)
Readily available (source / stocks)
Large numbers offspring per generation

Even though an organism can be well suited to being used as a model organism, there can also be some features that are not advantageous. For example, having similar proteins with different functions to that of humans mean that they cannot be used to study that particular proteins function. This may cause a problem during studies as it may cause anomalies in the results.

References;

David Botstein, Steven A. Chervitz, and J. Michael Cherry, 1997, Yeast as a Model Organism, Science, 277(5330): 1259–1260.