Modules are lightweight and flexible extensions used for page rendering. These modules are often “boxes” arranged around a component on a typical page. Some modules are linked to components, displaying information specific to or feeding information to that component. An example of this would be a "Report a problem" module that presents a form on every page for creating a ticket in the support component. However, modules do not need to be linked to components; they can be just static HTML or text.
Modules are meant to be small pieces of re-usable HTML that can be placed anywhere desired and in different locations on a template-by-template basis. This allows one site to have the module in the top left of their template, for instance, and another site to have it in the right side-bar.