How do I use Views with CiviCRM?
It is the regular Drupal Views module, but if you have Drupal and CiviCRM in separate databases, you have to edit your settings file to let Views know where to find the CiviCRM data. (Coleman notes in addition: "if your drupal sql user and civicrm sql user are different then you need to grant SELECT privileges to the drupal one for the civi db.") You can find the information you need to add to settings.php at Administer > System Settings > CMS Database Integration.

The wiki page has a pretty good tutorial on how to get Views working.

Then you can use default Views functionality on CiviCRM data -- for example:


set up a View for CiviCRM Participants, 
turn on Views aggregation, 
add the Event Title field (Aggregation settings: Group together)
add the Participant ID field (Aggregation settings: Count)