[My other notes from DrupalCon Denver]
Which entities are there?
In core, comments, nodes, user accounts, taxonomy terms, feeds.
In addition, contrib modules can define entity types. Modules that define their own entities include Drupal Commerce, Organic groups, Profile2, Heartbeat and Message, and File entity.
With Drupal 7, there is a shift from node-centric modules to entity-centric modules. This is the future for Drupal.