Entities in Drupal 7
A entity is data structure defined by hook_entity_info() and can be fieldable, which means that you can add fields to them.

In Drupal 7 Core, Nodes, Users, Comments, Vocabularies and Terms are entities.

In Contrib, there are many more, for example private messages and commerce as like a dozen of them ;)

However, I think job postings are perfectly fine as nodes :) Entities don't automatically have Pathauto (Tokens), Views etc. integration.