Module; This is a ‘container’ into which you can put many types of website feature.
For a module to show-up on the front end, you need to put something into it, assign it to a module position and assign it to the pages where you want it.

Just as a further note +2

Modules can be loaded into articles using the loadmodule joomla core plugin
{loadposition mypositionName}

Numerous modules are available from the very simple (login module or html text) to the more complex display of data coming from different sources

Modules are managed by the module manager page