What is the essential difference between docker and rkt?

Question

How are they functioning differently? Which features of the kernel are they using?

Answer 1

You can read all about it in this link

Basically, my impression is that rkt takes pride in being image-agnostic (meaning you can run images that were built using docker or other container engines) and contain less overhead than docker does. This is a nice picture to describe the differences between the two (taken from the link I've attached) -