Drupal is one of several PHP-based and MySQL database-driven content management systems installed on the EServer. It is a rather simple CMS--generally considered less powerful than Plone, but easier to design a collection with. It's very popular, and offers a similar number of open-source plug-in modules to customize and extend its functionality.
If you like analogies, Drupal is to Plone as Debian Linux is to OS X Server: the former come with a minimal install, which users are encouraged to build upon by finding or writing and installing numerous "plug-ins" that customize the capabilities to suit a site's needs. The latter come as a package with all needed functionality pre-installed and pre-integrated, inside a pretty user interface. By most writers' estimations Drupal installations are less powerful to customize (from a programming standpoint) than Plone, though somewhat easier to update at the source-code level (Drupal is programmed in PHP).
You can build an excellent, interactive webserver with either. For EServer editors who want to use Drupal for their collection, it is preinstalled on our servers and available for any editors who'd like to integrate it into their collections.
We have the following Drupal plug-in modules installed in our Drupal server (and we are happy to consider adding additional modules, if needed by our authors/editors):