The main tunable settings for PostgreSQL are in a plain text file named postgresql.conf that's located at the base of the database directory structure. This will often be where $PGDATA is set to on Unix-like systems, making the file $PGDATA/postgresql.conf on those platforms. This chapter mirrors the general format of the official documentation's look at these parameters at http://www.postgresql.org/docs/current/static/runtime-config.html. However, it is more focused on guidelines for setting the most important values, from the perspective of someone interested in performance tuning, rather than describing the meaning of every parameter. This should be considered a supplement to, rather than a complete replacement for, the extensive material in the manual.
Another live resource related to this subject is the article Tuning Your PostgreSQL...