Codes are what the cm enters into the computer to tell what kind of reservation you have. Generally here when people say code it generically means any kind of discount code that is out and about. All discount codes are limited in the number of rooms they are available for, have a booking window you must make your reservion in and a limited window of travel. And not all discount codes can be used throughout the disney resorts it is becomming more an more common that a discount code will be released but only be available at certain resorts.
So there are codes for -
rack rate
annual pass
Florida resident
General Public
Post Card Mailers
Each of these may have several more breakdowns to denote room only, package, package with dining, room category. They will also vary from season to season.
Post card mailers are person specific (only for the person/household they are addressed to). No one knows how you get them exactly. Suggestions are to be on any disney related mailing list and always say yes to being able to be contacted by whoever. These are usually very very good deals but not always.
For the others you have to meet whatever criteria there are have the annual pass, be a Florida resident. General public means that anyone can use the code.