An epic in Scrum terminology is a group of related user stories or may appear as a “big” story. Epics generally cover an entire use case or work flow for a feature related to user interaction with a system in software development. Epics are completed when all the associated stories implementing the epic are done-done. While stories may be completed and delivered independently, epics are only complete when all stories are verified done.
There is also a Scrum concept of a “theme” that is similar to an epic but does not encapsulate a specific workflow and does not need to have all of the associated stories completed to be considered delivered. Themes are generalizations for guidance, strategy, research and design.
User stories (or stories) implement features of a product or are portions of activity associated with a particular product epic and/or theme. User stories should be used to document and describe the activities necessary to implement the feature(s) that will satisfy the requirements described in the story as derived from the Product Owner (and requirements gathered). Generally, the writer of the story should include acceptance criteria motivated by the product owner.
In some cases, user stories will benefit from additional detailed information about the workflow / use case being described from a user perspective. Lists of steps, visual/document/other artifacts, business rules, use cases, et cetera if available should be documented in the story.
Any Epic may have one or more stories that implement the total scope of work of the epic. That is to say that in Scrum an “epic” is a group of related user stories. It is important to understand the communication structure of Scrum (an Agile) in order to effectively use the framework. While sizing and use of terminology/jargon in the industry is not settled there are general guidelines that can be derived from common sense interpretation of the Scrum framework. The key is to establish a consistent organizational communication using common terms and language.
UPDATE 2017 JULY 1 – The work chunks at the Portfolio is called what?
In order to confuse more people effectively, the Scaled Agile folks introduced the concept of an Epic at the Portfolio level in the SAFe. While I can see how an Epic (or theme) may exist at the portfolio level the prescriptive context of the portfolio (in my business experience) more naturally lends itself to the more traditional term “initiatives.” I believe themes are more closely relative to portfolio sized chunks of work, or initiatives. But, to each his own interpretation and jargon I suppose. I’m not a multi-millionaire framework owner just a blogger so my two cents are spent.