"Science is the systematic classification of experience." George Henry Lewis

The classification can be defined as the categorization of different elements that share characteristics, in order to have order, and to work easier with the whole set of elements. Classification for organisms is useful for studying each different organism easier and grouping the ones with similar characteristics. The classification of organisms has evolved with scientific discoveries and human experience.
The classification of organisms exist since primitive cultures needed it to survive, by knowing what was safe to use or eat, and what didn't. One of the most ancient documentation that exist about organisms classification is Aristotle's division of plants and animals. But with the progress of science, that classification was not enough and, little by little a much more complex and complete system of classification named Hierarchical System of Organism Classification. This classification normally includes 7 levels:

1. Kingdom
2. Phylum
3. Class
4. Order
5. Family
6. Genus
7. Species

However, super, sub and infra-groups may occur.


For each category in this system, a specific classification (that could be represented with keys) should be done. The following are examples of these:

http://faculty.southwest.tn.edu/rburkett/classification_of_organisms.htm