There are a huge number of writeups on E2 that I've discovered that are under the mistaken impression that
Solaris is
Sun's version of
UNIX. To avoid repeating myself in those nodes, I've decided instead to prepare this node for your reading pleasure.
Understand this above all things: Solaris is not an operating system. In other words, "Solaris" is not responsible for operating input and output devices, organising files, scheduling process time per CPU, or any of that. Those operations are taken care of by a kernel on any UNIX system. All versions of UNIX released by Sun Microsystems are versions of SunOS. The "Sun Operating System".
If you need proof, issue the "uname -a" command on any Solaris system:
% uname -a
SunOS sapda13z 5.7 Generic_106541-15 sun4u sparc SUNW,Ultra-Enterprise-10000
(Incidentally, Generic_106541-15 is the kernel patch level, in this case 15. 106541 is Sun's patch number for the SunOS 5.7 kernel.)
Solaris, on the other hand, comes bundled on two or three CDs. Solaris is the operating environment. This means that the term "Solaris" is used to describe all the software installed onto a system collectively. SunOS is one part of Solaris.
You can have a SunOS system that is not a Solaris system, but not the reverse. This is obviously cause for some confusion (evidence the number of nodes that describe Solaris as "UNIX"). Using Linux as an example with which I assume most will be familiar, consider "Solaris" to be on a par with te term "RedHat" and "SunOS" to be on a par with the term "Linux". You have a "RedHat system" in a similar that you have a "Solaris" system.
This comparison is not exact since Solaris was a term devised by Sun's marketing department to describe the entire software packaging that used to be simply "that stuff bundled with SunOS".
Corollary: redhat is not linux
Just to make things even more confusing, Sun have followed a seemingly erratic system of naming each release of Solaris. (Not unlike RedHat, I note with some amusement).
Rp makes an excellent point below. when it was released, "Solaris" did indeed move away from being a BSD style Unix and as a result it is very easy to distinguish between the two releases, hence the confusion.