I believe the extended codes are available on all-data etc, and are
available on a Genesis or other high end code reader.

There are, by some reports, 15,000 different codes.

The first 1000 "P" (as in P0234) codes are generic across all
manufacturers.  

After that, it's very manuafacturer specific, the same codes on two
different manufacturers cars can mean different things.

As well there are also "U", "C", and "B" codes.

If you want to see a list off all (well maybe not ALL) codes, go here;  
http://www.troublecodes.net/OBD2/

My personnal experiance indicates that all required codes for a give
vehicle are in the manufacturers Service Manual.  

The last time I had my car serviced (failed O2 sensor) the invoice actually
listed the code number, and description.  I don't think it's a big secret.

Scan Tools can be had for as little as $39.95 (Harbour frieght) or for many
1000's of dollars.  Not all tools will display all codes, you get what you
pay for.  

Oh and by the way, there are also several thousand commands that can be
sent to your OBDII computer, that will force the computer to do all sorts
of things, those are very rare to find anywhere, including the service
manual.

Do a google search on OBDII scan codes, for more information.