What is module? What can it do that a plugin cannot?
Modules, like plugins, allow you to create new front-end template tags, but they also allow much more powerful systems to be created.
Modules often have their own database tables, and can have an interface within the Control Panel, to add or modify data, or monitor the system’s behaviour.
They also allow you to capture the results of submitted forms and accept input from external systems through actions — special URIs that route requests to your site to the correct module for processing.
Modules are more complex to build — requiring multiple files and folders to be created — and they need to be installed from within the Control Panel.
Complexity: moderate to complex
Area of use: within templates as tags or form actions, within the Control Panel’s Add-on menu