What is the difference between $ and $$?

jQuery is an object provided by jQuery. $ is another, which is just an alias to jQuery.

$$ is not provided by jQuery. It's provided by other libraries, such as Mootools or Prototype.js.

More importantly, $$ is also provided in the console of modern browsers as an alias to document.querySelectorAll. Except if it's overridden by another library. $ is also provided in the same way, as an alias to document.querySelector.

See this answer for more information.