All living things belong to one of five kingdoms: plants, animals, fungi, protists, or monerans. These kingdoms are furthered divided up into sub-categories, according to their biological ancestry or differences in physiology:
Species is the smallest unit of classification and is used most often to identify certain organisms. A species consists of all animals that, under natural conditions, can breed and produce offspring that can eventually reproduce themselves. Many animals have several different species; for example, there are about 30 species of deer and 7 species of tiger. Some species are so similar to one another that they can breed and produce fertile offspring, but they rarely do this in the wild, and may also occupy separate geographic ranges. Some species are divided into subspecies based on differences in appearance, such as variations in color for some bird species. Subspecies can interbreed but usually do not do so in nature, most commonly because they live in different geographic regions. Extinction occurs when all the individual members of a species die.
Scientists sometimes find it easier to talk about extinction in terms of larger categories, such as families. And a family can include anywhere from two to many thousands of species. For example, the giant panda is so different from all other species that is has a species, a genus, and a family entirely to itself. If this rare creature becomes extinct, a whole family of life will have vanished from the earth.
Scientists who study life-forms of the past (paleontologists) estimate that between 90% and 99% of all species that have ever existed are now extinct. So for every known living species today, almost 100 species have disappeared. Perhaps the best-known symbol of extinction are the dinosaurs, who vanished from the earth 65 million years ago.