when and how to use pgsql system catalogs?

Hello,
Could somebody tell me when and how to use pgsql
system catalog tables (those start with pg_)? I read
the chapter 43, and know they store database meta
data,and what each table does, but I am not quite
sure when and how to efficiently use it.
btw, which command allows me to see all those tables?

Thanks and Regards,
William