A mythology is a collection of myths or stories about a specific person, culture, religion, or any group with shared beliefs. Most people don’t consider mythology to be entirely true, but they still take it seriously.
A myth is a story about the olden days, often featuring supernatural characters, and a mythology is a bunch of myths that are related to each other. Greek mythology is filled with tales about relationships between gods and humans, usually with gods pulling pranks all the time. Christian mythology tells the story of God creating the Earth and everything after. Mythology can also mean the study of myths in an academic situation, like studying mythology at a university.