The first kind of modules are used for creating applications. You could build your own blog or CMS (in fact there is already some kind of simple CMS system build for the system (called content - which is used on this site for writing the - you get it - the content!). There is also a simple blog system. And a tutorial about building a blog.

Modules are placed outside webspace. Modules can be loaded as shell modules or web modules.

Templates are another kind of module: Templates are placed in webspace, which makes them great for placing html, css, images, and also PHP files (in some cases). Then you can make available some Javascript or CSS for other templates (or regular modules). And you could glue it together with a couple of PHP functions or a class.

The third type of modules are profiles. Profiles are just a simple php file with a var_dump of all modules and templates, module version, and git repos, and also some ini files for all modules. Then you can distribute your profiles (usually web applications) in a easy manner. Just clone a profile. All modules and templates will then be cloned when profile gets installed.