Where can I find a list of internal airline designations for various aircraft/cabin configurations? Is there any way to tell what the internal designations of an airline mean? Looking at Delta's seat maps for the 757, Delta has a couple of kinds, but what's the difference? 


Boeing 75A
Boeing 75M
Boeing 75S
Boeing 75G


I know they're slightly different configurations for different routes, over water etc, but can what type is used on what route/configuration or what the designations mean be found somewhere?   

Is there a place where these are defined for airlines of the world?
Internal designations are usually used to differentiate various seating configurations or differences internally for an airline. From your example, all those codes are various codes for the 757-200 (at Delta, they code the 757-300 as a 75Y). You can often look up various seat configurations at Seatguru.com, a site that generally tracks seating information on airlines. Additionally, all would be sent to ATC using the ICAO type B752.

The reason for these variations can be broken down to a couple areas of differences. One is exit configuration; in the same type (757-200), there can be 2 or more different configurations of emergency exits (combinations of doors and windows, and various types). If you have a fleet that mixes these, you'll often have to have different seating configurations for them. Another differentiation could be whether there is a subfleet equipped and used for ETOPS flights. There can be other reasons an airline could want a subfleet and a different cabin. Also, if you're in the process of upgrading a fleet, you might want to create codes to differentiate them so you know which is going where and be able to plan your revenue models around that.