There are not sub-species of humans because we have not had enough time to evolve enough genetic differences. On different continents on earth, people have evolved slightly different characteristics from each other due to selection and genetic drift. Different races of people have different skin color, others a greater ability to store fats (eskimos), others have evolved different blood types (sickle cell), etc., however, this differences are not big enough to classify us as different subspecies. Naturally, subspecies evolve very slowly. Humans have only had 100,000 years to evolve differences from each other, which has not been enough time and I doubt humans will ever evolve subspecies because there is not global mixing of genes through inter-racial partnerships.

Subspecies can be created in animals in less than 100,000 years through artificial selection, which is what humans have done with domestic animals. Because humans can been able to choose genetic characteristics and animals have a relatively short time to reproduction, we could get very genetically different breeds in a matter of tens of generations.

There are - the different ethnic groups of humans qualify as subspecies. There used to be more subspecies, but they've become extinct.

That said, humans are remarkably uniform in their variety; most subspecies are more different from each-other than the human subspecies are. The reason for this is not completely known - although in the case of dogs, dogs were bred artificially, so their breeds are much more different than what you get in nature.

Actually, breeds of dogs are all the same species (and the same goes for cats)! If they were not the same species, they would not be able to produce "viable offspring," or offspring that could also reproduce. So if you want to carry the analogy across to us, we do kind of have "breeds" in the sense that people from different parts of the world have different "phenotypes" (physical characteristics). I hope this helps!