What is a species?

Historically the term “species” has been used to denote a rank in the taxonomic classification system, typically to denote organisms that can interbreed, or organisms that appear to be similar. However, the definition never worked completely, as many things that were classified as different species could actually interbreed (maize and oats, for example). Furthermore, there is no consensus definition of the term or common criteria for identifying new species, so the term can be subjective. This confusion has increased with the availability of DNA sequence data, as organisms that look different may actually be genetically quite similar, while those that look to be closely related may turn out to be quite genetically distinct.

For more info, see:
Stanford Encyclopedia of Philosophy. http://plato.stanford.edu/entries/species/