Custom modules are esentially standard ASCX controls / Web parts (very similar technology is used in SharePoint, but MonoX does not have its heavy footprint). MonoX infrastructure frees you from worrying about the basic tasks (authentication, authorization, Web part configuration, Web part communication...) and brings some rather advanced stuff (personalization, various providers, caching, optimization, ...). It also comes with dozens of parts that are ready to use in your projects.
There are no e-commerce functionality built in it at the moment, but we will gradually publish some of the modules that were built for our custom projects. We have also used various integration strategies to use MonoX with other ASP.NET e-commerce packages (nopCommerce was our primary choice).