Auditing is the capability of performing introspection over an application or user session, in other words, to be able to reproduce, step by step, what the user or the application asked the cluster to do.
Auditing is slightly different from logging, as logging provides a simple way of saving whatever action of the user, but without providing an easy way to reconstruct the user or application interactions with the cluster. In fact, in a highly concurrent cluster, many actions made by different users will coexist in the logs in a mixed bunch of lines. Moreover, logging does not provide any particular logic on what it is storing, and therefore it becomes hard to find out what a user has done. This becomes even more true when the user or the application executes complex statements, in particular, statements where parameters and values are not explicitly provided.
As an example, consider the following simple section:
forumdb=> PREPARE my_query( text ) AS SELECT * FROM...