Drupal is one of many Content Management Systems (CMS) out there. It's good for building sites with robust functionality. With drupal, you can build membership sites, forums, helpdesk ticketing sytems, company directories, online invitations sites, e-card sites, and more. It was designed with this in mind, from the beginning, and consists of a core base, to which you can add modules, or extenstions, if you will. There is a module for almost any type of function you can imagine, and if there isn't, you can build a custom one, or ask a developer to build one for you. Pretty cool, right?