What do you mean by plugins, modules and components used in Joomla?
Plugins enable users to execute code in response to certain events, custom events or core events of Joomla that are triggered by users own code.
Components are the prime function units displayed in the template, like the content management system, web links, contact forms and likewise.
Modules are lightweight and flexible extensions used for page rendering. They are used for the small bits of the page which are generally less complicated and able to be seen across different components. This is one of the best Joomla Interview Question of almost every interviewer. Must read it multiple times.