- Species: unique things (animal or plant) that do not breed breed with each other. Of course there's more to that when the species does not have sexual reproduction.
- Subspecies: a subdivision of species that can reproduce with each other, but may not due to either geographic or behavioral differences.
- Breed: a man-made subdivision of animal species that typically can and do reproduce with each other unless humans prevent it.
- Cultivar: a man-made subdivision of plant species that do not reproduce with each other. Often, cultivars are unable to reproduce without human intervention. An example is that broccoli and cabbage are cultivars of the same species of plant (brassica oleracea).
- Morph: a subspecies of animal that do not interbreed due to behavior.
- Strain: in microbiology, a subspecies of bacteria or virus with unique characteristics, such as the H5N1 strain of avian influenza; in animals, especially lab rats, a subspeces specifically bred with certain traits, such as diabetes.
Feel free to correct me if I'm wrong, and I'll update the original post.