Descibe plugin system in D8 and its usuage in Drupal core?

Plugins are small pieces of functionality that are swappable. Plugins that perform similar functionality are of the same plugin type.

Drupal contains many different plugins, of different types. For example, 'Field widget' is a plugin type, and each different field widget type is a plugin. The admin user may select from the list of field widget plugins to set the widget that a field uses.

The D8 plugin system provides guidelines and reusable code components to allow developers to expose pluggable components within their code and (as needed) support managing these components through the user interface.

Modules define plugins: a module may provide plugins of different types, and different modules may provide their own plugins of a particular type.

Plugin Example:-

1. Block
2. Field Type, Field Widget, Field formatted 
3. Text Filter 
4. View 
5. Migration Source