Let’s go through the various agile artifacts and their hierarchy.
A theme is a collection of epics and user stories used to help prioritize and visualize the application features. Themes help in roadmap planning of the product.
An Epic a story or feature that is too big to be completed in one sprint. Epic would be broken down into smaller stories, tasks, etc.
A user story is a high-level requirement of the application. From the end-user perspective, a user story is a requested functionality, written in plain English. The user story is a thin slice of the application providing value to the end-user business when complete.
Sprint task a piece of work required to make or transform the story into done state.