There has been much debate about what makes a story a myth but there is a rough consensus that a myth deals with gods, heroes and supernatural beings and events rather than with the stuff of everyday life (unlike a folktale, where the main characters are often ordinary people).
Myths also tend to address fundamental questions about the world and humanity and why they are as they are, for example: How was the world created? Why do the seasons change? Why is there death and sorrow in the world?
The universality of these questions may provide part of the explanation as to why there is such a similarity between the myths of different cultures. They are rooted in the fundamentals of what it means to be human and our utter dependence on the natural world.