I will explain this using the same wording as Atlassian.
An epic captures a large body of work. It is essentially a large user story that can be broken down into a number of smaller stories. It may take several sprints to complete an epic. More info.
A story or user story is a software system requirement that is expressed in a few short sentences, ideally using non-technical language. More info.
A task is a unit of work contained within a story. More info.
If your epic would be: As a user I want to see a comments section at the bottom of the website. Your stories would be: 1. As a user I want to read existent comments. 2. As a user I want to leave own comments. And finally your tasks would be: 1. Create a textbox for comments. 2. Create a database for the comments and user data. 3. Integrate a button for saving comments.
I hope this answers your question. If you have further questions, just let us know.