What is the difference between a species and a sub-species?
From a biological standpoint, a species is the largest unit of potentially and actually interbreeding organisms (i.e. two different species never interbreed, if they did, they would be one species).
A subspecies is a group of organisms inside a species that interbreed and share some unique characteristics that differentiate them from the others in a species (i.e. benthic and limnetic stickelback fish).