This Joomla tutorial goes over the basics of modules and positions. The content layout of a web page designed in Joomla! is all about positions and modules. Much like a newspaper editor lays out a newspaper page in blocks, positioning each element of the page, the Joomla! designer publishes blocks or modules in defined positions on the page.
Modules are usually lightweight extensions containing blocks of content that appear in “positions” defined by the template.can be published on just certain “pages” (determined by menu items) or on all “pages”.