How does a subscriber unsubscribe from my list/newsletter?
A subscriber can unsubscribe by clicking a link in the email / newsletter.
Link Example
The subscriber clicks the link in an email or newsletter.
User Subscription Management
The link opens this window. The subscriber clicks the Unsubscribe button.
Confirmation
Example Goodbye Message
A message is sent to the subscriber confirming the action.

If you want to be notified when someone unsubscribes, follow the instructions in this post.