Drush is the Drupal Shell utility. It allows Drupal developers to control many aspects of a Drupal installation through command line commands. Drush allows for easy installation of new projects on a Drupal site, including downlading and enabling the projects on the site. Drush also allows for easy installation of 3rd party libraries into the sites/all/libraries folder. Outside of core Drupal related functions, Drush also has become popular with contributed module developers, many of which now include Drush commands with their modules. There are also a growing list of Drush specific modules that add additional functionality to Drush at it's core. Because of this, Drush has become one of the most powerful tools that a Drupal developer can have in their toolbox outside of Drupal itself.
I will be cataloging various aspects of Drush on this page, including Drush commands, Drush modules, and useful Drush information.