1) CBM is Conceptual Balance Mod. Basically, the Illwinter developers (both of them) did not make gameplay balance a priority. Thus in MP, there are a lot of units, pretenders, and spells that do not make much sense to buy or use. CBM, developed by quantum mechanic (sp?) attempts to make a bunch of stuff otherwise not useful, useful. The number of chances is absolutely enormous and makes the game close to unrecognizable from "vanilla" Dominions 3. However, it is popular for MP.
2) As you may have noticed, there is a sub forum dedicated to mods and maps. Most mods and maps are uploaded onto the forum and made available for download there. Or, go to
www.llamaserver.net and click on Maps and Mod Browser. This will give descriptions and images and provide links to where the mod or map can be downloaded from the forum.