A subspecies is an individual division within a species, meaning that the members of the sub-group are individual enough that they cannot be lumped together, but they are not so distinct that they are entirely different species. This biological specification can be a bit confusing, and the categorizations are constantly being adjusted and changed to deal with new information. As a general rule, you can think of it as being almost like a race of individuals, although human races are not considered subspecies, for a variety of biological and political reasons.
Before delving into what makes a subspecies, it may help to think about the taxonomic rank of “species.” A species is an individual class of organisms that are distinctive from other animals, and unable to breed with other groups of animals. Members of a species are defined on the basis of their differences, which may be quite diverse or very minimal. For example, you can probably think of a lot of differences between an elephant and a pet cat, because these animals are considered to be different species, but there are also differences between Indian and African elephants that lead them to be classified as being in different species, despite superficial resemblances.
Many people think of “species” as the terminal taxonomic rank, since it does imply a sense of finality. In fact, many species are extremely diverse, and as a result, the smaller rank emerged to do justice to this diversity. The different subspecies within a species are distinctive and unique from each other, but they are still capable of interbreeding.
One example that you may be familiar with is the domestic dog, Canis lupus familiaris, as opposed to the wolf Canis lupus lupus, or the dingo, Canis lupus dingo. All of these animals can interbreed, but they are morphologically distinct, and they lead very different lives. Within the genus of Canus, lupus is considered to be a “polytypic species,” meaning that it has a number of subspecies; a species with no smaller divisions is called a “monotypic species.”
There are a number of ways to indicate a subspecies. In the method used above, animals are identified as Genus species subspecies. You may also see “ssp” or “subspecies” in binomial nomenclature, like this: Panthera tigris ssp. altaica, for the Siberian tiger.