Modules are ways to expand the core of Inkscape in order to provide additional functionality. Modules don't imply anything about implementations or anything else really. Module is a generic term to describe that this 'thing' isn't part of the core. Many modules may be implmented in C and compiled in, while other may be other programs on the user's computer wrapped so Inkscape can use them.
This Wiki page aims to be an initial starting point for writing modules, and understanding how they fit into Inkscape. If you'd like to write a module you should probably start looking at this page and its subpages for information. If you want to write a module and you don't have any ideas here are some: [PlugIns Module Ideas]
A module is described my a [ModuleDefinitionFile Module Definition File]. This file is a XML file which tells the basic information about the module (e.g. name, author) and also gets into the details of what this module does and how it does it.