A phylum (FYE-lum) is one of the main groups scientists use to organize living things. When biologists study the incredible variety of life on Earth, they group organisms by their basic body plans and structures. A phylum is one of these major categories.

Think of it like organizing a massive library. Just as books get sorted into big categories like fiction and nonfiction, then broken into smaller groups, living things get organized too. The animal kingdom contains about 35 different phyla (that's the plural). Each phylum contains creatures that share fundamental similarities in how their bodies are built.

For example, all animals with backbones belong to the phylum Chordata. That includes fish, birds, reptiles, mammals like us, and amphibians. Animals with jointed legs and hard outer skeletons, like insects, spiders, and crabs, belong to the phylum Arthropoda. Soft-bodied animals like octopuses, clams, and snails belong to the phylum Mollusca.

What makes organisms part of the same phylum isn't whether they look alike on the surface. A whale and a salamander look completely different, but they're both chordates because they share that basic blueprint of having a backbone. Scientists use phyla to understand how different life forms are related and how they evolved over millions of years.