An urban myth is a made up story that spreads from person to person by telling. The person telling often says they know someone who knows someone that the story happened to. Urban myths are sometimes based on truth, but have been greatly exaggerated. There may be different versions, but the teller will assure the listener that the story is true. There are so many of these myths that collections of them have been published.
One common urban myth is about a crocodile/alligator that has been released into the sewer system under a city. The crocodile/alligator grows and grows. Another urban myth is about a woman who had a growth on her face. When the growth pops, it is full of tiny baby spiders. Neither of these stories is true.