Drush is a command line shell and Unix scripting interface for Drupal, a veritable Swiss Army knife designed to make life easier for Drupal power users who spend a lot of time hacking away at the command prompt.

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.