Skeletal systems allow organisms to stay upright, move around, protect organs from injury, provide support, and serve as reservoirs of organic and inorganic substances.
Invertebrate skeletons belong to organisms without bones, especially backbones. Many invertebrates (including all arthropods) possess a skeletal system on the outside of their body, known as an exoskeleton.
DAT Pro-Tip: think of exoskeleton as exit (outside)
Vertebrate skeletons belong to organisms that do have a backbone and they are known as endoskeletons because they are found within the body.
Endoskeletons can be separated into:
Adapted from: https://commons.wikimedia.org/w/index.php?curid=6881901 and https://commons.wikimedia.org/w/index.php?curid=6881947
Analogy: If an endoskeleton were a tree, the axial skeleton would be the tree trunk and the appendicular skeleton would be all the branches coming off it.