Subspecies is a classification (taxonomy) in biology. It is directly below species. When looking at the Latin name, subspecies are indicated by the third name. Subspecies can mix with each other; animals or plants from different subspecies can have offspring together (This is usually not the case with animals from different species).
Very often, there are populations that are between two subspecies; this is because evolution is a continuous process. Subspecies can often be recognised by subsp or ssp (before the third part of the name) especially in botany.
Examples of subspecies are:
In zoology, when a species is split into subspecies, the first described population is known as the "nominate subspecies", and it repeats the same name as the species. For example, the "Daboia russelii russelii" (known as the Indian Russell's Viper) is a nominate subspecies of the species "Daboia russelii" (known as the Russell's Viper).