Shadowing involves observing the operation of dental practice and occasionally standing behind a dentist and watch him do his thang and try not to get in his way nor the assistant's.
In volunteering, you pretty much do nothing but clean the office armed with lysol and windex. Maybe if you get experienced enough, they may let you do something dental related--like sterilizing instruments, aka dish-washing.
Sometimes, one gets real lucky and gets to volunteer under a really caring dentist who will guide you and teach you couple things such as mixing various cements, how to chairside assist, etc etc in which case you can slowly began assisting.