In his book Scrum, Jeff Sutherland, defines two types of items to help you define your goal. In the first phase of the project design, users create an epic and then break it down into user stories.
In the case of Amazon.com, the epic might be, "As a customer, I want an online bookstore so that I can buy any book I want, at any time, and have it delivered to my door".
This epic is then broken down into stories:
Story 1: "As a customer, I want to browse books by genre, so that I can find the type of books I like."
Story 2: "As a customer, I want to add books to a shopping cart, so that I can buy them all at once."
Story 3: "As a product manager, I want to track my customers' purchases, so that I can market specific books to those users based on past purchases."
Breaking it down further, tasks and bugs are used to help shape these stories. For Story 1 above, two types of task are created:
Task 1: Create each genre and classify all of the books into those genres.
Task 2: Insert a filter field.
Bug: The search functionality doesn't accept the characters 'A' and 'E'.
The story is successfully met when the team members complete the tasks and bugs in the story. When they can't complete a particular task or bug in the story, they can move the items to the backlog and take it up in the next sprint.