Modules are plugins loaded on demand at runtime to provide additional functionality. They are somewhat wrappers/bindings/adaptors to offer access to functionality your application or a library of your application likes to expose to scripting backends.
I think its unnecessary to enumerate the possible implementation styles:
Modules are plugins loaded on demand at runtime, and expose functionality of an application or library to scripting backends.
--Argonel 05:15, 18 January 2007 (CET)