Body plan: the overall organization of an animal's body.
Scientists group all animals according to the similarities in their body plan. A body plan is the overall organization of an animal's body: what it looks like; what kind of internal and external parts it has and where they are located; how it moves and feeds. A group of animals with a similar body plan is called a phylum. Organisms within a phylum have a shared evolutionary history as well, meaning they are "related" to one another through a distant common ancestor. Invertebrates are so diverse that they include dozens of phyla (plural). In contrast, vertebrates, animals with backbones, are only a small subgroup within one phylum.
Most commonly seen invertebrates belong to one of these six phyla. If an animal belongs to one phylum, it does not belong to any other phyla.
We can learn to recognize all these different groups by the similarities in their body plans.