Most people have a basic idea of what species are, even if they are not sure of the best way to define the word species. Quite simply, species are kinds, or types, of organisms. For example, humans all belong to one species (the scientific name of our species is Homo sapiens), and we differ from other species, such as gorillas or dogs or dandelions. But defining, identifying, and distinguishing between species really isn't that simple. In fact, it is often a complex and difficult process-especially in cases of new or previously unknown species. Biologists frequently disagree about species, and even argue over how best to define the word species. This disagreement is so well known, and so much discussed, that it is sometimes referred to by biologists as the "species problem" (Hey 2001).

This article explores the idea of species, including both the meaning of the word species, and how biologists think species can be identified in nature. It also examines why an understanding of species is important, both for the study of biology and for our society.

The central difficulty when studying species is that, even though all species are kinds of organisms, all kinds of organisms are not species. For example, birds are a kind of organism, but birds are not a species --there are many thousands of species of birds. For scientific purposes, it is not enough to identify a kind of organism. As a biologist you must also determine what level or rank of kind to assign to an organism. If you discover a new kind of organism then you must decide if it should be called a new species, or if it falls within an already described species. For example, the common chimpanzee species, Pan troglodytes, appears to include several slightly different kinds of chimpanzees. Each of these have been given the rank of sub-species. Alternatively, a newly discovered kind of organism might be so different from other known species that it receives not only a designation as a new species but also a ranking as a new genus.