What are User Stories and Epics?

User Stories are high-level functional requirements, mainly used in Agile projects.
A user story typically has 3 parts-

• AS A - this identifies the user - it could be a general user, for example, the customer browsing the website, or it could be a specific set of users (business class users) or it could even be a system. The user/actor defines WHO wants the requirement.
• WANT - this refers to the action. WHAT is required? For example, I want to cancel the order, I want to open a new account, etc
• SO THAT - this refers to the WHY section, i.e, the advantage or use of the requirement. 

Some examples of user stories are

• The customer can order gift cards online using their credit cards
• As a patient, I should be able to make a reservation for a doctor online, so that I can see the doctor without waiting

User stories are very high-level requirements, that can be used as the starting point to flush out more details of the requirement. User stories are easy to write than use cases and functional requirements and it is typically written by stakeholders or business analysts. 

Good user stories would have the following characteristics - Independent, Negotiable, Valuable, Estimable, Small and Testable.

Epics are larger user stories that can be further broken down into several user stories. Epics are usually lower priority user stories, which when later is taken for development, is examined more closely and divided into several user stories and may be completed in more than one iteration