Species refers to a group of organisms that is isolated from another group in terms of reproduction. The members within the same species share a common gene pool. This enables them to reproduce offspring that remain fertile.
Subspecies are not isolated in terms of reproduction. They are sufficiently varied to be termed a subspecies but could still breed and produce fertile offspring.
Species exist as populations. It is populations that evolve, not individual beings.
Species is often referred to in the abbreviated form "sp." or for the plural form "spp.". The term species is a basic unit of biological classification and a taxonomic rank.