Up to date as of October 29th, 2015.
In the introduction to content and configuration translation piece we discussed what is considered content in Drupal 8. This is a somewhat misleading term because custom blocks, custom menu items and even user entities (user profiles) are considered content in terms of their implementation.
Content is always stored as entities. The main difference between configuration and content entities is configuration is usually created on the backend (think views, vocabularies, etc.) while content is usually created on the frontend (think free tagging taxonomy terms, comments, blog posts). This is not a black and white differentiation but it helps think of the categories. The other key differentiator is content entities usually get to have configurable fields. You can add new fields to user profiles, taxonomy terms or comments. Again there are exceptions, for example custom menu items cannot get configurable fields in core. Finally, there are even content entities that will not be stored, in Drupal 8 contact form submissions are content entities that live only until they are sent via email. For this tidbit we are concerned for content entities that are stored and multilingual.