The statment FROM ubuntu:14.04 means use the ubuntu image as a base image.
The ubuntu image is not an OS. This image “mimics” an Ubuntu OS, in the sense that it has a very similar filesystem structure to an Ubuntu os and has many tools available that are typically found on Ubuntu.
The main and fundamental difference is that the Docker Ubuntu image does not have it own linux kernel. It uses the kernel of the host machine where the container is running.
Moreover, the size difference between the Docker image (73MB) and an Ubuntu iso(around 1Gb) is very significant.