COD, or Conference Organizing Distribution, is an installation profile for Drupal, that makes it easy to create a conference website with features like event signup, session proposal, session voting/judging, session selection/scheduling, and social networking prior to events. Read more about COD's features on usecod.com and https://www.drupal.org/project/cod
For the latest information related to the process of porting this installation profile to D8, be sure to see https://www.drupal.org/node/2605232
For additional information about the D8 profile, including terminology we use, see https://github.com/drupal-cod/cod/blob/master/README.md
An event is all the conference sessions in one particular timeframe, e.g., Drupaldelphia 2016, vs. Drupaldelphia 2018. Grouping by events allows conference organizers to create repeating events, while reusing locations or timeslots from previous events.