Arthropods are creatures, which in life have an exoskeleton also called a carapace- or shell.
They have jointed limbs and segmented bodies. Segmentation was a development which allowed an animal to specialize and compartmentalize areas of it's body. It allowed the animal to feed and clean itself and do all kinds of special activities. Walls called septa divide off areas of the body from others and act sort of like doors from one room to the next. The jointed limbs of the animals allow them to move.