Understanding conversation biology requires an understanding of the taxonomy used to organize the wonderfully diverse organisms of this planet.
Species are the broadest set of organisms able to successfully interbreed, subspecies is a further classification of a species, but in order to fully understand the relationship we must take a look at scientific naming. Binomial nomenclature is a fancy word for the system used to give all living organisms, from trees to bacteria, their 'latin name', the difficult/impossible to pronounce two name system written in italics, such as Homo Sapiens . Homo being genus, and sapien the species. The common name being, of course, 'human'. Species is more specific than genus. However if scientists need to further separate species into groups, they use an even more specific term, the subspecies. According to the International Code of Zoological Nomenclature, subspecies is the only taxonomic rank under species. Not every animal is classified into subspecies.
A species is typically defined as a group of organisms capable of interbreeding and producing fertile offspring. For the most part, this is the guideline taxonomist follow. For example, a horse and donkey can interbreed to form a mule (female horse and male donkey) or hinny (male horse and female donkey). However, because the differences in genetics both mule's and hinny's are sterile. Thus, donkeys (Equus asinus) and horses (Equus caballus) are two separate species. Tell me science is'nt crazy! That a horse with 64 chromosomes can mate with a donkey that has 62 chromosomes, and something pops out! Crazy right.
A subspecies is further classification for a species. Subspecies can interbreed and form perfectly normal adorable offspring. However while they "are capable of interbreeding and producing fertile offspring, they do not interbreed in nature due to geographic isolation or other factors." This leads to natural adaptations and changes, subspecies will often look and behave differently. Take for example, the dog and the wolf. Both can interbreed successfully and so are classified as the same species Canis lupus. But while your Pomeranian (Canis lupus familiaris) could theoretically breed with a Eurasian wolf (Canis lupus lupus) this would likely never happen. In the case of the domestic dog, Canis lupus familiaris, Canis is the genus, lupus the species, and familiaris subspecies. Understanding these simple facts can help make sense of otherwise confusing scientific articles.