There are many ways to get Drupal running on your Mac. OS X comes preinstalled with PHP and Apache, so theoretically, you'd only need MySQL. Acquia makes a separate version that includes Drupal, Apache, MySQL, and PHP (aka DAMP) already set up. This version is great if you really want to get up and running quickly. It's tied to the Acquia distribution of Drupal which has some extra features that we don't tend to use for our development.