Taxonomy is the science of identifying, classifying, and distinguishing different species. Once you have a basic knowledge of taxonomy you can predict behavior patterns in species of marine animals, and make decisions about their suitability in your aquarium with some degree of certainty. It will allow enable you to keep animals that are not regularly in the trade and expand the overall aquarist knowledge base. For example, this is a picture of Strombus luhuanus.
Its species name is luhuanus, its genus name is Strombus.
All Strombus spp. are herbivores. The Strombus genus contains many species of what are commonly called "conchs" or "true conchs". When you see an article or an item description that says "Strombus spp.", the author is referring to more than one species of conch, that is what the extra "p" in "spp." stands for. When a person is referring to a specific species (a possibly unknown one though), or just a singular specimen they will use Strombus sp. instead, it is short for "Strombus species".
Strombus spp. belong to the Strombidae family. A family is the taxonomic rank above genus. Spider Conchs are also members of the Strombidae family, but their genus is called Lambis. It is typical to refer to Lambis and Strombus species as "related" because they are in the same family even though they belong to two different genera. As it turns out, all Strombidae are herbivores.
Knowing similar dietary habit across a taxonomical family can be important, especially when common names do not align with taxonomic categories. For example, the animal commonly known as the "Crown Conch" is not a member of the Strombidae family. Its species name is Melongena corona. All members of the Melongena genus are predators, like all species in the Melongenidae family. In fact, the Crown Conch preys on Strombus spp. true conchs. Wholesalers in the hobby are still unaware of this distinction and continue to sell Crown Conchs to unsuspecting fish store owners.
Family and even higher level taxonomical ranking information can still be a good starting point to understanding a species you are unfamiliar with. Members of the Oxynoe genus look like nudibranchs, which are notoriously difficult to keep due to their particular feeding requirements. However Oxynoe spp. are members of the Plakobranchidae family which are in the herbivorous order Sarcoglassa. Nudibranchs are in the order Nudibranchia. Since both nudibranchs and sarcoglassans hitchhike their way around in the hobby, knowing to distinguish the two may help you determine whether the strange creature on your live rock is friend or foe.
Once you get above the rank of order behavior among the species represented begins to diverge remarkably. Every species discussed so far is a member of the class known as Gastropoda. They comprise all land, freshwater and marine snails, slugs and limpets. Therefore knowing the class of a specimen is a start to identifying it, and is less useful in indicating its behavior. If you have an unknown hitchhiker that has a single shell (or evolved from a species that had one and is now a slug) it is likely a gastropod. If the animal has two shells, it is a bivalve, in the class Bivavlia. These include clams, oysters and scallops. If it has 8 plates to form a protective layer similar to a shell, it is in the class Polyplacophora, which comprises the chitons. Here it is the general form of the shell that distinguishes the species apart.
Above the rank of class you have phylum. You will make most of these classification distinctions without much thought even if you are not familiar with the particular phylum name. Mollusca for snails and clams, Arthropoda for crabs and shrimp, Cnidaria for corals and jellyfish etc...One exception to this are tunicates, which often resemble sponges but have a central nervous system during their life cycle and are in the phylum Chordata - with all the other vertebrates like the Indian Elephant and you.
Sometimes it can be easier to distinguish between different phylum, than between kindoms. Kingdom level distinctions in the hobby are mainly used to distinguish between plant life in the form of seaweed, and animals in the form of sessile invertebrates. Encrusting coralline algae and certain macroalgae/seaweed can resemble coral animals and hydroids. I learned this the hard way after touching a stinging hydroid I thought was a species of macroalgae.
Beyond kingdom is the highest living taxonomic rank, domain. In aquaria it comes into play when you are distinguishing between film algae which are Eukaryotes just like the three toed sloth, and cyanobacteria, which is a bacteria.
There are other taxonomic distinctions that make the whole system messy. Subphylums, Superfamilies, and even unranked classifications such as Clades that help taxonomists classify a specimen. The most notable of these to the hobby is variation. It is abbreviated as "var." in a species name. Variations of species are published when specimens of the same species have significant differences. What amounts to enough difference is a matter of debate each time a new variation is named. Pictured below though can see a fairly stark example. The two specimens are identical species, but one is Caulerpa cupressoides var. lycopodium.
The difference in expression of these genetically identical species is thought to be from location and exposure to the elements. Because we are trying to recreate the environment these species naturally come from in our home, this information can be handy. C. cuspressoides comes from calm water, while its lycopodium variation comes from rougher seas. Variations can be found in fish and coral, although the different variations of corals that get names like "Rasta Zoas" or "Miami Vice Zoas", aren't two different taxonomical variations of the same species, they just look a little different.