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 SunOS4.1.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 SunOS5.x, which is an SVR4-derived UNIX, along with OpenWindows 3.x, tooltalk, and other stuff. (See 1.5 for a chart with more info)