A distro is an operating system built on the Linux kernel. A kernel is useless on its own and needs and operating system around it to do something. Just like the NT Kernel has Windows, the Linux kernel gets an OS too. Each OS built on Linux is called a "Linux Distribution or Distro." Each distro represents a full operating system. Examples of Linux based operating systems (distros) would include: Linux Mint, Ubuntu, Fedora, RHEL and CentOS, Suse, OpenSuse, Arch, Gentoo, Debian, etc.
Not sure if I completely agree with this. I would define a distro as an OS with a custom configuration and additional software installed. These are the things that make them unique.
They all have the same basic OS components. The kernel, some sort of interface and hardware support. They may be different versions, but same components.
The different shell's (BASH, CSH, etc), GUI's (Gnome, KDE, etc) service managers (systemd, SYS-V, etc), package manager (yum, apt-get, etc) and may other things are what makes each distro different from the rest.
But then, this is just my opinion.