Solaris(tm) is Sun's name for their UNIX-based user environment,
including the UNIX(tm) operating system, window system (X11-based),
and other stuff too.
Solaris 1.x is a retroactive (marketing?) name for SunOS 4.1.x
(x>=1), a version of UNIX that is BSD-like with some SVR4 features,
along with OpenWindows 3.0.
Solaris 2.x (which is what most everybody means by "Solaris")
includes SunOS 5.x, which is an SVR4-derived UNIX, along with
OpenWindows 3.x, tooltalk, and other stuff.
Solaris 7 and later are basically newer revisions of Solaris 2.x
with the leading "2." stripped.
This FAQ covers Solaris 2.x and later.
(See 1.5 for a chart with more info)