In laymans terms......The niddy noddy (assembled tool) is for wrapping spun yarn into a skein, the nostepinne, top thicker arm, is for wrapping the yarn into a ball, the spindle, with one of the whorls attached, is for spinning fleece into the yarn. The WPI gauge in the center is for measuring the ply of the yarn. Number of wraps per inch gives it's thickness. The whorls are different weights giving the drop spindle a heavier or lighter drag on the fleece as it is spun allowing the spinner to make thicker or thinner yarn.
Hope that helps not confuses. I often get asked to make something and have to go and research it to find out what it is etc. Makes a change from bowls and platters LOL.
pete