The fine folks at MySQL provide a complete list of MySQL errors:
http://dev.mysql.com/doc/mysql/en/Error-handling.html
If you receive a MySQL error message, check out the MySQL site for a detailed explanation.
If you have shell access (*nix) or access to the mysql\bin directory (Windows) you can find out what the error code means by using the perror utility:
http://dev.mysql.com/doc/mysql/en/perror.html