There are several definitions for species.
A species has several features that are not shared by other groups. This is the oldest definition which is solely based on "visible" features. The main problem with this definition is the question how different to groups have to be before they can be considered as species.
The Evolutionary species concept
A species descents from a common ancestor and shared evolution.
The biological species concept
A species is a group of (potentially) interbreeding individuals that are reproductively isolated (in place, time or behavior) from other groups. All members of the group share a common ancestor. The species is the fundamental unit of evolution, while higher taxa (families, orders, ..) are artificial groups reflecting a possible evolutionary relationship.
The Genotypic cluster definition
A species is a common genetic pool that are identified by their genetic differences from other groups. This is the latest definition which is based on differences in DNA sequences.
Varieties (var.) are often used to recognize the existence of local variants, although the choice between subspecies and varietal status in some groups is sometimes more a matter of personal taste than of adherence to a strict definition.