TUTOS has a module concept which helps "outside" devolpers to integrate there work into the existing framework. All the integration is done via configuration files and an API that new modules must use.
If you like to write your own module it is a good idea to read the stuff below AND to have a look at the existing modules.
The later subsections will describe additional, but common, features you can, or should, use in modules (like own table config handling and language files)