1.000: The AIX operating system - what is it?

This is best answered by reading the text files in /usr/lpp/bos. The
README file there contains general information and the bsd file contain
useful information if you know BSD and/or System V.

The last release for the RT PC is 2.2.1. The latest release for PS/2s
and Intel architecture machines is AIX 1.3; for PS/2s only, 1.2.1. For
the RS/6000, there are five major levels, 3.1, 3.2, 4.1, 4.2, 4.3, and
various intermediate levels. IBM has dropped support for 3.1, 3.2, 4.1
and 4.2

Y2K compliancy issues have been addressed by PTF's on 3.2, and 4.*. 

For those who don't have a copy of /usr/lpp/bos/README or bsd, I'll
sumarize: IBM tried to follow IEEE, POSIX 1003.1, ANSI C, FIPS and
then X/Open Issue 3.  Beyond that, AIX is a combination of System V
and BSD.