Hi,
mbn - Stands for MAIN firmware files
fbn - Stands for FILE SYSTEM firmware files
cust pack - Customization packs - Stands for regional, country or operator configurations

MBN contains (as it says) the main firmware code itself. All the basic funtions of the phone are here.
FBN contains the files that you find navigating the phone files (ringtones, pictures, etc). All the files of the phone are here. May change from country to country and operator to operator.

Custom packs contains the configuration files to make your phone work well in your country or to make your operator lock the phone, etc. As it says, it customizes your phone.