What is a subspecies?
In practice, subspecies are often fairly loosely-defined, reflecting a degree of uncertainty and ambiguity at this level of taxonomy. There are systematists who take the view that subspecies-level classifications should not be used as they are not rigidly definable.

In general, a subspecies will fulfil the following criteria:


it will occupy a distinct geographic range from other subspecies
within its species, 
it will have limited gene flow to other subspecies within its species, while still being capable of interbreeding
it will have some recognizable shared character, used to distinguish it from other subspecies within its species


Subspecies classifications, and their usefulness, are discussed in the Turtle Taxonomy Working Group's 'Turtle Taxonomy: Methodology, Recommendations, and Guidelines', for those interested in further reading.