What is the difference between `null` and `undefined` in JavaScript?

Imagine a parking space. `undefined` is a space that has never been assigned to anyone — it's blank on the booking sheet. `null` is a space that someone has intentionally marked as 'available / empty' after the previous car left. Both are empty, but `undefined` means 'nothing was ever put here', while `null` means 'I deliberately set this to nothing'.