Subspecies (commonly abbreviated "subsp." or "ssp.") in biological classification is either a taxonomic rank subordinate to species, or a taxonomic unit in that rank (plural: subspecies). A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one. (However, all but one subspecies may be extinct, as in Homo sapiens sapiens.)
Organisms that belong to different subspecies of the same species are capable of interbreeding and producing fertile offspring, but they often do not interbreed in nature due to geographic isolation or other factors. The differences between subspecies are usually less distinct than the differences between species, but more distinct than the differences between breeds or races (races can be assigned to different subspecies if taxonomically different). The characteristics attributed to subspecies generally have evolved as a result of geographical distribution or isolation.
Read more about Subspecies: Nomenclature, Criteria, Monotypic and Polytypic Species