This is a super simple step by step guide on how to install Drush on a Mac:

1. Download the latest Drush version from http://drupal.org/project/drush.
2. tar -zxf drush.tar.gz 
3. sudo mv drush /usr/lib/
4. chmod u+x /usr/lib/drush
5. sudo ln -s /usr/lib/drush/drush /usr/bin/drush
6. Type drush in your command line! ;-)

Drush?

Yeah, Drush is Drupal on Shell.

More here: http://drush.ws/