An operating system, at least a Unix like one, is made of a kernel which interfaces the hardware, a C library, the libc which interfaces the kernel and the userland, standard utilities and libraries and and a set of applications usually including a graphic environment.

Linux is the kernel
Gnu/Linux is a very minimal Unix like OS that almost all Linux distributions are leveraging on.
Distributions includes in addition to Gnu/Linux, things like KDE or Gnome, Firefox, Apache, MySQL, package management, whatever.

Solaris is then similar to a Linux distribution. It includes a kernel (SunOS 5.x), libraries, utilities, one or more graphic environments, applications, etc ...