Arcs

This is an image from the Animators Survival Guide, a good book for studying motion through arcs and curvature. Every joint in a body creates an angle when it is bent or extended, creating measurable degrees, much like using a protractor. Joints create a hinge type motion, limiting e.g. an arm to swing in an arc,but always attached to a fixed point (the shoulder)...
...But joints come in all shapes and sizes, so while a limb has long bones between each joint, a spine, tail or hair ismade up entirely of joints, so it would create more of  a wavey 's' effect, able to curve further than an elbow/knee joint (which when applied to an arm or leg creates a slapstick movement, much like charlie chaplin)