SitecoreDXG was specifically designed to make it easy for developers to extend and customize it to suit their needs, and the most straightforward way to make modifications and customizations is through plugins.
Plugins are independent node modules with their own dependencies that follow a specific structure in order to satisfy a particular role of the SitecoreDXG ecosystem. There are two types of plugins supported by SitecoreDXG, natively: Trigger Plugins, and Completion Handler Plugins.
Plugins are necessary because while SitecoreDXG was specifically built to be easily integrated into any CI/CD pipeline or automated process, there are always things that end-users might want to do differently or control based on their individual requirements. By enabling developers to write their own trigger and completion handler plugins, SitecoreDXG can be customized to suit nearly any need.
By definition, plugins are technically components, though they could be better described as sub-components of the SitecoreDXG Generation Service.