Drush stands for Drupal Shell, and is often referred to as the "swiss army knife” for Drupal developers. With the latest release ("Drush 2") there is no longer a contributed module. This makes Drush 2 independant of Drupal, whereby functionality is available from the operating system (e.g. Linux) command line. Drush 2 uses an autodetect mechanism, whereby Drush 2 is able to determine if you are in a directory associated with a Drupal installation, with commands directed to that Drupal installation.
Before upgrading modules with Drush, be sure to back up the Drupal database and the Drupal directory.
A related module is the Drush System Manager, which provides the ability to list variables, cck field, and view information from the command line. These output listings can then be checked into version control, so that changes made to content types and views can be tracked. See http://drupal.org/project/drush_sm.
drush help => List drush commands
drush dl views => Download the Views module
drush enable views => Enable the Views module
drush update => Update all contributed modules
drush updatedb => Update database (same as update.php)
drush cache clear => Clear caches
drush cron => Run cron
drush statusmodules => generate list of module info