Plugins are swappable pieces of code in Drupal 8. To see how different they are from hooks, let's take an example where we want to create a new field type.
In Drupal 7, this involves:
Providing information about the field
hook_field_info- describes the field, adds metadata like label, default formatter and widget.
hook_field_schema- resides in the module's .install file. Specifies how the field data is stored in the database.