Evowiki on species - good brief discussion

Mayr on species, Giraffe species

Extreme variation within species

Species concepts

Species concepts - advantages and disadvantages of different concepts

Observed instances of speciation

Species - the basic unit of evolution
Biological 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