Agile Overview → Work Items
A work item is anything that is used to build and track work during any phase of a product's lifecycle, including high-level features (epics), a more detailed work item (such as a story and its individual tasks), issues raised by customers or members of your test team (defects), and issues that prevent team members from completing their assignments (impediments).
In general, a work item lives in backlogs (product, release, and sprint). As a work item move into a smaller backlogs they should decrease in size, but also increase in detail. A work item is configurable and can have many different names. Their relationships (to each other) are defined by your organization's work breakdown structure.
Serena Agile Planner uses epics, stories, tasks, and defects. In general, an epic is the largest work item. Epics should broken down into well-defined stories (which then contain specific tasks). Defects are like stories (in that they should be well-defined and that they may contain specific tasks). An impediment is the final work type. In general, impediments can be resolved quickly; should your organization need to create impediments that require actual work by actual team members, you should add a user story into a backlog that is used by the team that will do the work that is needed before the impediment can be removed.