Scientists tend to like to organize things into groups and perhaps the most obvious example of this is the classification of living organisms by biologists. Since organisms were encountered by man they have been divided into groups, for example into plants or animals, according to their features. These features were initially structural features that could be seen by the human eye: “rodents” covers many species of animals but all of them look similar with fur, a long tail, large incisors and many other characteristics. Later more sophisticated techniques allowed classification according to molecular biology: you can determine how similar or different two animals are by looking at their gene sequences (the more similar the gene sequences the closer they are related).
The common classification system used by biologists is hierarchical. Initially all living organisms are divided into five Kingdoms: Prokaryota (bacteria), Protista (protozoans, algae), Fungi, Plantae, Animalae. These kingdoms are further subdivided, first into Phyla, then Class, followed by Order, Family, Genera and Species in that order. (The mnemonic KP Crisps Onion Flavor Get Smelly can be useful to remember the order.) To illustrate this with an example: humans belong to the Kingdom Animalae, Phylum Chordata, Class Vertebrata, Order Primates, Family Hominidae, Genus Homo and finally species Homo sapiens.
This classification system is actually linked in to the order that scientists believe that evolution occurred. Assuming a single common ancestor the first divergence of species would have been into the five Kingdoms. The original Animalae then diverged into Protostomia and Deuterostomia and Deuterostomia then diverged into the Phyla Chordata and Echinoderma. (Protostomia ancestors eventually gave rise to insects, worms and molluscs among other species.) The Chordata then diverged to give rise to animals with a backbone (Vertebrata) and without a backbone. Eventually the Vertebrata diverged into many orders etc. This evolutionary order is continually being confirmed as the genomes of animals are being sequenced.
The advantages of this organized system are firstly, that it promotes clarity (a bird may be called one thing in one language and something different in a second language but its biological nomenclature will be the same for everybody). The second advantage is that it allows scientists to easily identify similar species which will aid scientific research: scientists know that rodents are fairly closely related to humans and so they make good models of human disease etc. in scientific experiments.