Think I’m not breaking any news for most of you guys when I say that Drush is the best thing since Drupal itself. But if you still don’t know it, here's a quick definition:

“A command line shell and scripting interface for Drupal.”

Was it helpful? No? Ok, lets try to simplify a little bit more. Drush makes developing and maintaining your drupal site easier by providing you with an array of terminal scripts for common tasks on drupal such as, clearing cache, downloading modules, etc.