Drush is a command line shell and scripting interface for Drupal that allows users to perform administrative tasks and common commands from the terminal instead of through a web browser. It can be used to download and update Drupal core and modules, enable or disable modules, clear caches, run cron jobs, and more with just a few keystrokes. Drush requires a Linux server with PHP CLI and shell access, and can be installed by downloading a tarball or checking out from CVS and then configuring drushrc.php files.