Tuning Variables For PostgreSQL

I read all the time that most DBA's are required or should tune their
DBMS which obviously in my case would be PostgreSQL but I'm curious
what exactly is involved when tuning a DBMS like PostgreSQL. What are
some of the basic functions involved when tuning? Are there specific
things I should tweak on a newly built server freshly installed with
PostgreSQL? Can someone please clarify what exactly most people do
when they "tune"? Lastly I'm sure this has been discussed but after a
Google search, I can't find any updated info since 2009 so I would
like to know specifically what file system you've found PostgreSQL to
work on the best? What file system will give me the best performance
and stability on disk? I've currently running it on Linux Ext4 file
system and have no had any issues but I was wondering if there was
anything out there more suited to perform better on PostgreSQL.

Thanks!