Species can be defined in many ways and it is both a classification and a concept.
The Linnean classification system labels species as a taxon. A classification to which individual organisms can be grouped. The evolutionary concept of species should not be confused with this Linnean classification. The same word defines two different concepts. The evolutionary concept of species consists of a group of closely related individuals that are connected in history by ancestral relationship. It is not necessary to use the Linnean classification to define the evolutionary species (Wiley, 1980). In fact it is merely so that the Linnean convention is more convenient on a daily basis.
A species is a group of organisms that share enough characteristics to be distinct from other groups of organisms and this group of organisms interbreed forming an interlinked group.
E.O Wiley (1980) 'Is the Evolutionary species fiction? A consideration of classes, individuals and historical entities.' Systematic Zoology, Vol 29:76-80.