Evowiki on species - good brief discussion
Mayr on species, Giraffe species
Extreme variation within species
Species concepts - advantages and disadvantages of different concepts
Observed instances of speciation
Species - the basic unit of evolutionBiological species concept: Species are the members in aggregate of a group of populations that breed or potentially interbreed with each other under natural conditions (Futuyma, 1986)The above (Futuyma) is a good start. Like all definitions is has problems, for example groups of animals that lived at different times can't be unambiguously assigned. Organisms that are interfertile, but don't interbreed (e.g. the 3 living zebra species) are separate species by this definition. If you take it too literally, Great Danes and Chihuahuas are different species.
Sibling species - are genetically isolated but difficult to tell apart.
An interesting example of cryptic species