the various major bond rating agencies such as Dominion Bond Rating
Service, Moodys and Standard and Poors have their own nomenclature(s)
for classifying bond risk.  Mainstream media often reports changes to
such ratings as if everyone knew what they meant (like an NFL point
spread).
Please tell me what the classifications are in descending order from
AAA (I presume that really is the highest)  and what each class means
in layman's terms.

Hi! Thanks for the question.

I have found the following explanations to the bond credit ratings of
the major agencies. There are still some business terms but they are
fairly understandable.

Moody?s Ratings
http://www.treasurer.ca.gov/ratings/moodys.htm 

http://www.aztreasury.state.az.us/investGlossary.shtml 

http://invest-faq.com/articles/bonds-moody-ratings.html 


---------------------------------
Standard & Poor's
http://www.treasurer.ca.gov/ratings/sp.htm


--------------------------------
Fitch
http://www.treasurer.ca.gov/ratings/fitch.htm 


--------------------------------
Dominion
http://www.dbrs.com/web/jsp/pub_ratingscale_cp.jsp 


Our next links provide additional explanations that can help you.

Fiscal Advisors - Bond Rating Definitions
http://www.fiscaladvisors.com/rating_defs.asp


Search terms used:                           
Dominion Moody?s Fitch "Standard & Poor" "bond rating" definitions
                 
I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if any of the definitions are
unclear to you or you have further questions or if you would need
further information.
                         
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Easterangel-ga                         
Google Answers Researcher

I asign your answer a AAA rating.  Thanks.
It is also interesting how the various rating agencies use somewhat dissimilar 
gradations.

The 00 rating signifies that Bond has a license to kill.

leapinglizard

Thanks for the kind words, the 5 stars and for the tip!