Drush is a computer software shell-based application used to control, manipulate, and administer Drupal websites. On the surface, Drush is a tool for updating site modules, however, Drush has a more comprehensive list of features.
Introduction to Drupal Rector: A Tool for Automated Code Refactoring and Drupal Version Upgrades
Drupal Rector is a tool that helps developers upgrade their Drupal websites to newer versions of the platform and automate code refactoring tasks. It is designed to make upgrading and maintaining Drupal websites easier by automating many of the manual tasks required when upgrading or refactoring code.