Drush is a powerful shell for Drupal. It can many developer tasks really easy to manage from the command line. If you are installing it on Mac OSX, there might be a step or two which you have to perform for Drush to work correctly.

If you have pear or wget on your machine, then the process would be straightforward as described on the Drush site. However, if you are manually downloading the Drush then:

1. Download and untar the Drush in a directory outside your Drupal.

2. In addition, you might have to get the Console_Table. Download and untar it. Find the Table.php in the newly untarred directory. Copy it to drush/includes as Table.inc (i.e. rename the file).

Step 2 is required when you get the following message while running Drush:

Drush needs a copy of the PEAR Console_Table library in order to function, and the attempt to download this file automatically failed. To continue you will need to download the 1.1.3 package from http://pear.php.net/package/Console_Table, extract it, and copy the Table.php file into Drush's directory as /Applications/MAMP/drush/includes/table.inc.

Hopefully this solves your problem if you see the above error.