Chapter 32. The Information Schema
  The information schema consists of a set of views that contain
  information about the objects defined in the current database.  The
  information schema is defined in the SQL standard and can therefore
  be expected to be portable and remain stable — unlike the system
  catalogs, which are specific to
  PostgreSQL and are modelled after
  implementation concerns.  The information schema views do not,
  however, contain information about
  PostgreSQL-specific features; to inquire
  about those you need to query the system catalogs or other
  PostgreSQL-specific views.