When the requirement of the client is given to the developers, both the developer and stakeholder agree upon a unit of work which is called as “User Story”. User stories are the best way to collect data about a feature to facilitate conversation between client and team. It is the smallest unit of work also known as ‘task’.
User Story Format:
“As a [type of user] I [want/can/am able to/need to] so that [reason]”
Themes may be defined as a collection of related user stories. Big user stories are known as Epics. Generally, the stories are related to each other and are connected to a particular customer. Some stories might also be dependent on each other but it is not necessary to get it delivered together nor has to follow a specific work flow.
Epics are also made of multiple stories. Epic often defines complete work flow for a user. Epic needs to be broken down into many smaller stories before development team could work on it. The major difference between themes and epic is that the stories that consist of epic may be completed independently but these do not have a business value until the whole epic is complete, while stories comprising a theme are related, each is independent enough to be delivered separately and still has some business value.
So, story, epic and themes are just terms use to simplify the conversation that the scrum team have and it can be used as an industry standard.