Written literature is often based on oral stories. Before humans developed writing, we passed down stories in the form of oral literature. With the invention of writing, people began to record these stories. In these stories they passed down their culture, traditions, values, and histories. Many early stories were part of religion. For example, myths and legends were passed down this way, including the creation myths of different cultures.
Storytelling is still very important. Many of our modern stories can trace some of their influences to ancient stories, but oral storytelling still continues today. Whenever you describe something funny that happened to you, you are telling an oral story. If you write it down later, or turn it into a book, you have completed the cycle of oral literature becoming written literature.