A "phylum" is a major category in the biological classification system used to organize living organisms. It groups together organisms that share a common body plan or significant structural features. For example, the Chordata phylum includes all animals with a backbone, such as mammals, birds, and fish. Phyla are one of the higher taxonomic ranks, sitting above "class" and below "kingdom." This system helps scientists understand the relationships between different species and their evolutionary history. For instance, the Arthropoda phylum includes insects, spiders, and crustaceans, all of which have jointed limbs and exoskeletons.