Taxonomy

Taxonomy is the system or organization for the millions of living things on the planet.  Organisms are divided into groups based on physical characteristics and/or evolutionary lineage.  Those groups are then divided into other groups until each organism is on its own.  Those groups are called: 
Kingdom
Phylum
Class
Order
Family
Genus
Species

There is a "scientific name" for each creature, consisting of the names of it's bottom two levels (genus and species).
An "identification key" can be used to find the scientific name of an organism.