The term species has undergone many changes in the history of evolution. It has always been used to describe groups that were intrinsically different. Originally, the distinction was based on morphological differences. However, it soon became apparent that some types of organisms had different forms at various stages in their lives. Additionally, some organisms came in different forms, such as the queen and worker forms found among many social insects. These individual types of insects were clearly the same type of organism even though they were morphologically different.
Because of these problems, the modern definition of species does not mention morphological similarities. Instead, it focuses on reproductive ability. Today, a species is defined as a group of organisms that shares the same gene pool and can successfully mate. As is discussed in more detail in Reproductive Isolation, the emphasis on "successful" reproduction is both important and problematic. Many closely related species can produce infertile offspring, while in other species populations exist that cannot mate with each other but can each mate with a third population.
Species can also be seen as populations between which there is interbreeding or gene flow. Whether populations that have the potential for interbreeding but for some reason do not interbreed (for example, because of geographic isolation) can or cannot be considered the same species is a point of contention among biologists. For our purposes, we will consider a species to be a group of populations that can interbreed to produce viable offspring.