Inside an image

I was wondering what goes into a base Docker image, such as an Ubuntu 14.04 image. I’m new to the whole concept of containers and I haven’t thus far grasped how it’s possible to run different GNU/Linux distributions. Do all distributions share a common kernel and any differentiating components are fit into the image that is containerized? What about older versions of distributions that were meant to use older kernels?

It would be much appreciated if someone could either answer this or guide me to a place that has already answered this, because I couldn’t find a suitable answer in the documentation or somewhere like Stack Overflow.