A CMS has two components: a content management application (CMA) and a content delivery application (CDA). The CMA is a graphical user interface that enables users to design, create, modify and remove content from a website without HTML knowledge. (google)
In terms of WordPress, Joomla, Drupal, only Joomla uses for a kind of extension the word component, abd can be FREE or PAID.
But in general component is a WordPress’ plugin or a Jommla’s extension or Drupal’s module.
A Drupal module is a collection of files containing some functionality and is written in PHP. Because the module code executes within the context of the site, it can use all the functions and access all variables and structures of Drupal core.
A WordPress plugin is a piece of software that “plugs into” your WordPress site. Plugins can add new functionality or extend existing functionality on your site, allowing you to create virtually any kind of website, from ecommerce stores to portfolios to directory sites.
An extension is a software package that extends your Joomla! installation in some way. A small selection of extensions is included with the default Joomla! installation but many more are available from the Joomla! Extensions Directory
also the non popular CMS’ have free or paid components