Difference between geckodriver and marionette

I don't fully understand the difference between geckodriver and marionette.

• Can anyone clarify the difference between them?

For example, when I use Selenium WebDriver to control a Firefox browser, I need a geckodriver binary to listen the WebDriver protocol from Selenium.

• Is Marionette the Firefox extension that actually drives Firefox?

Selenium uses W3C Webdriver protocol to send requests to Geckodriver, which translates them and uses Marionette protocol to send them to Firefox

Selenium<--(W3C Webdriver)-->Geckodriver<---(Marionette)--->Firefox