Ubuntu is the most popular Linux distro which handled by canonicals. It offers several installations which is customized such as gnome-ubuntu, ubuntu-core. Ubuntu core is a light version of ubuntu which is not a installation iso but its a file system to work with linux in minimal resources.
After you setup the Ubuntu core you get only the file system with Ubuntu repos, shell, grub and other initial items for linux. It is very good to try this out because this gives you a good knowledge of the linux file system how format the disk, how to install grub, how make the disk bootable, how will kernel interact with grub and so on.
Firstly you have to download the Ubuntu-core rootfs (root file system). You can download the ubuntu 14.04.3-core root file system in this link http://cdimage.ubuntu.com/ubuntu-core/releases/14.04/release/
