The delegator is the user who initiates a workflow and invites users to review, they are also responsible for assigning feedback to authors to complete.
The reviewer is a user that has been invited to review a story on workflow, e.g. a client. They can only access the preview of the story and is the area where they will give feedback. If the user is also an author they can access the story through the preview or story editor.
The author is anyone in your team that is able to create stories. The delegator will assign authors to complete feedback on stories.