To be short
An entity type is a base class
A bundle is an extended class
A field is a class member, property, variable or field instance (depending on your naming preference)
An entity is an object or instance of a base or extended class
Entity types
In earlier versions of Drupal, the field system was only used on content types. Now, thanks to the Entity API, we can add fields to other things, like comments. An entity type is a useful abstraction to group together fields.
Some examples of entity types:
