There are currently five types of module:
Stand-Alone Module Edit
Stand-Alone modules are just that - you can insert any of them into your world and they will work right away, they're completely self-contained and require no other modules to function. Currently, the majority of the Gamemode 4 modules fall under this category.
Base Module Edit
Base Modules don't do much by themselves, rather they provide a base platform that other modules can use and communicate with.
Reliant Module Edit
Reliant modules act like stand-alone modules, adding new elements to your gameplay, but require a base module to properly function.
Expansion Pack Edit
Expansion Packs add functionality or new elements to another module. Expansion Packs are also reliant on other modules.
Module Pack Edit
Module packs are similar modules bundled together to save space and reduce lag by sharing clocks etc. They are designed to reduce lag if people wanted all of the modules in the pack and to make their installation faster.