You might have heard the term "GUC" before, especially on the Postgres email lists. It is documented as an acronym meaning "Grand Unified Configuration", but the section it references doesn't mention the term "GUC" at all. Pretty obscure, huh?
Our Grand Unified Configuration (GUC) system was created in 2000 as a way to control Postgres at various levels. You are probably most familiar with controlling Postgres via the postgresql.conf file or using the SQL SET command, but it is much more powerful than that.