The Washington Hunter's Ed Course has a section dedicated to the parts of a shotgun. It has an interactive animation that shows you each of the parts as well as their function and placing on the gun.
The animation showed on the sight is a picture with a list of parts on the side. If you click on any of the parts from the list, the picture zooms in on that specific part while the information regarding the shotgun part appears in the text box below. The description of each part talks about its function or functions when using the firearm.