Drush is a command line shell and Unix scripting interface for Drupal, a veritable Swiss Army knife designed to make life easier for those of us who spend some of our working hours hacking away at the command prompt.
A few core Drush features:
* The Drush Package Manager allows you to download, enable, disable, uninstall, update modules/themes/profiles/translations from the command line in a very simple way (apt-get style) - just type e.g. drush dl views and drush enable views in a Drupal directory to install the Views project!
* Additionally, the Drush Package Manager also allows you to update all your modules with just one command - drush update.
* Drush Core: several useful utilities for site administrators and developers (e.g. drush cron; drush cache clear; drush sql dump)