There are several terms which we use to describe requirements:
What are the formal definitions of these terms and what are the differences between them (except volume of described requirements)?
If there are some other terms to describe requirements, please, tell me about them.
Each organization and author has a different definition for these terms.
In this article, Mike Cohn provides his definition of User Stories, Epics and Themes:
A user story is simply something a user wants. User stories are more than just text written on an index card but for our purposes here, just think of user story as a bit of text saying something like, "Paginate the monthly sales report" or, "Change tax calculations on invoices." Many teams have learned the benefits of writing user stories in the form of: "As a <type of user> I <want/can/am able to/need to/etc.> so that <some reason>." But it is not necessary that a user story be written that way. Check out the advantages of that user story format.
A Scrum epic is a large user story. There's no magic threshold at which we call a particular story an epic. It just means "big user story." I like to think of this in relation to movies. If I tell you a particular movie was an "action-adventure movie" that tells you something about the movie. There's probably some car chases, probably some shooting, and so on. It tells you this even though there is no universal definition that we've agreed to follow, and that an action-adventure movie must contain at least three car chases, at least 45 bullets must be shot, and ….
So, "epic" is just a label we apply to a large story. Calling a story an epic can sometimes convey additional meaning. Suppose you ask me if I had time yesterday to write the user stories about the monthly reporting part of the system. "Yes," I reply, "but they are mostly epics." That tells you that while I did write them, I didn't get the chance to break most of them down into stories that are probably small enough to implement directly.
Finally, "theme" is a collection of user stories. We could put a rubber band around that group of stories I wrote about monthly reporting and we'd call that a "theme." Sometimes it's helpful to think about a group of stories so we have a term for that. Sticking with the movie analogy above, in my DVD rack I have filed the James Bond movies together. They are a theme or grouping.
During the sprint, tasks are defined for each user story, so that the development team can have a clear sense of how it will accomplish its work. Tasks are simply more granular versions of the work entailed to complete a user story. A task is a technical piece of work necessary to get a story done. Developers split a story in technical tasks to get a realistic estimate of the time it will take to complete a story. It is important to note that user stories are usually estimated using story points, whereas tasks are estimated with hours.
A feature is a distinct element of functionality which can provide capabilities to the business. It generally takes many iterations to deliver a feature. A user story is a part of the feature. By splitting a feature in smaller stories, the user can give early feedback to the developers to issues quickly.
The relationship between epics and features is the most controversial in the agile community. In my opinion, an epic is a broader concept and an epic can be broken down into different features.
These definitions are good general guidelines, but your individual team may have a different interpretation and that's okay. It is more important that your team has its own agreed upon definitions:
A Feature is a chunk of functionality that delivers Business Value (revenue, engagement, etc.) that probably contains multiple User Stories and/or Themes.
A Theme is a group of User Stories with similar characteristics.
An Epic is a User Story that takes more than one Sprint to complete.
A User Story is a tool used to create a conversation between the Dev Team and everyone else so the developers can estimate how much effort will be needed to deliver a specific outcome. It is usually:
- A high-level description of a desired outcome/result
- Completed in one Sprint (generally)
- Broken down into a series of Tasks (measured by # hours or half days)
