Drush is a command line shell and Unix scripting interface for Drupal. Drush core ships with lots of useful commands for interacting with code like modules/themes/profiles. Similarly, it runs update.php, executes SQL queries and DB migrations, and misc utilities like run cron or clear cache. Developers love the generate command, which jump starts your coding project by writing ready-to-customize PHP and YML files. Drush can be extended by 3rd party commandfiles.
The Drush project expects all participants to abide by the Drupal Code of Conduct.
A: The Drupal Shell.
Some people pronounce the dru with a long 'u' like Drupal. Fidelity points go to them, but they are in the minority. Most pronounce Drush so that it rhymes with hush, rush, flush, etc. This is the preferred pronunciation.