A simple description of Drupal is that it's a great CMS (Content Management System), social networking and publishing application, forum, and blogging tool - the more complete answer is that Drupal is a framework written in PHP for quickly and reliably building all kinds of powerful web applications.
The UK Drupal community organises events, helps clients find local Drupal developers, enables local Drupal developers keep in touch, discusses UK specific issues, and generally encourages Drupal connections from Land's End to John O'Groats.
DrupalCon Europe takes place in London in August 2011, with around 2,000 delegates expected to attend. More details will be announced in the coming months.