From Envirowiki

Species (plural: species), is a the lowest classification of life form. For example, Humans (Homo Sapien) are a species of the order primate. Common names for animals and plants usually refer to a specific species, but can sometimes refer to a family of species.

A species can be defined as a group of organisms that can inter-reproduce. Diversity in species (biodiversity) comes though evolution.

The hierarchy of life classification runs thus:

Life > Domain > Kingdom > Phylum > Class > Order > Family > Genus (Sub-genus) > Species (Sub-species)