Beings from different origins
Sometimes, mythology and folklore are used interchangeably–but there is a difference between them.
Just looking at their definitions will not tell us much. Trying to come up with a relation between them doesn’t help much either–you can have mythology being a subset of folklore or vice versa depending on who you’re asking.
But there are some things that separate them when diving a bit deeper.
What they contain
In mythology, we are mostly hearing about gods and things related to them. There is a hero that is the child of a god that needs to prove himself, or an actual god that is out to do something or another. Creatures and monsters from mythology are often there to try and stop a hero or help them on some rare occasions.
Folklore, on the other hand, talks about magic and monsters without involving any gods at all. The hero of these stories is often an everyday man–a farmer, woodworker–out not to prove himself but to do his job. Beings are in these stories to represent both dangers and hope
What they explain
A myth is often created to explain something larger than life; world creation, why we have four seasons, how day and night work, etc. They are more like a religion than just stories, explaining our existent.
Folklore is stories that either explain the origin of a local event or place or explains why a place is dangerous. Why wandering in the forest can be dangerous, be careful around waters (bridges, rivers), etc. It is explanations to everyday occurrences such as causes for diseases and how to cure them.
When they originated
I’m not sure this is true for the general mythologies and folklores, but it is true for the ones originating in Scandinavia.
In the Nordic countries, folklore is what mythology developed into when the old gods got replaces by the single one of Christianity. The people still needed magical creatures to explain the local things, but the larger than life aspects got replaces by the Christian religion.