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 ...
Linux is nothing but a 5-megabyte file that the bootloader loads on startup. It's capable of running applications but doesn't have any. Everything else, everything, is whatever applications distribution X wanted to bundle with it or user Y arranged to have installed. So the kernel amounts to a utility for running other programs.
Solaris a package deal, kernel and applications and everything. So is any Linux distribution, though Linux doesn't really belong to any distribution the way Solaris does to Sun/Oracle.