Drush is a command line tool that allows interaction with a Drupal site. The tool itself is incredibily useful and provides mechanisms to download modules, backup databases and most other things that can be done with Drupal. Drush Make was a plugin for Drush which has now become part of the Drush core and allows Drupal sites to be created via a make file. What Drush Make does is to use the make file to download the modules, themes and libraries needed for a Drupal site ready for the site to be installed. This means you can give this make file to another developer who can then build their own Drupal site.