As we all know, Joomla! modules are those small widgets you can place on a Joomla! page. Their execution is independent of the component (which controls the main content) of the page and are independent of each other. They can be activated and deactivated on each individual page. Joomla! modules provide supplementary functions that are otherwise not available through the main component, or may supply functions of one component while the visitor is navigating on another component. That means you can have a Joomla! module that is providing a VirtueMart function while a visitor is browsing a com_content article.
One typical use of Joomla! modules is to provide a menu or supplementary menus. In this regard, mod_mainmenu is probably the most important Joomla! module. Another important Joomla! module is the mod_search module which provides a search function while the visitor is navigating through the pages.
VirtueMart is actually a bundle of applications...