The word epic usually refers to the brave action or excitement or the films about the deeds of gods and great man. From it's literal meaning, we can conclude that user story epics is a great, large and complex set of user story. User role must be included in the epic user story description to picture the real scenario on how they will use the system. There are 2 types of epic user story; compound and complex. Compound user story can be dissambled into smaller user stories within the same story subject while complex user story is an extremely large and generic stories that cannot simply be broken down into smaller stories.
The "chunk" of splitted user stories shares the same subject and is called a theme.
To get clearer picture, look at these example:
Epic
- User can cancel hotel reservation
Theme
- Privilege member can cancel reservation up to last minute
- Normal customer can cancel not less than 48 hours
- Customer can cancel reservation by phone