what is smooth scrolling

I fail to understand the difference between auto scrolling and smooth scrolling. I have some issues with the scrolling experience on Firefox but I think it improved today and felt happy. Then I read that enable smooth scrolling by default has been changed on Aurora recently. But failed to understand the difference between smooth scrolling and auto scrolling. I find scrolling in Chrome and Safari to be very aesthetic and nice but never found the above two scrolling types in them.