What are bundles? Can we create custom bundles?
Bundles are an implementation of an entity type to which fields can be
  attached. You can consider bundles as subtypes of an entity. With
  content nodes, for example, you can generate subtypes like articles,
  blogs, or products. Not all entity types have bundles, however. For
  example, users do not have separate bundles or subtypes. With bundles,
  you can have as many subtypes as you want. Then, using the Field
  system, you can add additional fields to those bundles such as file
  download field for Articles and location fields for Users.


Read more here and a good article that i found. Hope it helps :-)