These are my own definitions, not official:
Shadowing - purely observing. Usually close by the pharmacist seeing what tasks they complete throughout the day. Potentially accompanying the pharmacist in counseling/MTM sessions for observation if the patient allows.
Volunteering - usually completing a task that will help contribute to the overall system, but not necessarily observing the pharmacist. You could be restocking shelves, drawers of vials/caps, taking out the trash, shredding, etc.
Sometimes, you'll be shadowing and asked to help out with a task (as described above with volunteering).
Both are great experiences to put on a resume and to get you a feel of pharmacy.