Cache Questions

First off I would like to preface this question with the fact that I am new to the concept of caching outputs.  I have heard of it before, but just now scratching the surface on how it all works.  I know that it can speed up load times by not making continuous SQL queries and the like.  My question is how it all really works in the punbb scheme of things.  Is there a fixed expiry scheme in place?  I would look a the source, but I am unable to download the files at my place of work.  I had a brain fart while on my lunch break for some plugin/mod ideas and was wondering about using pre-existing pun functions (if they exist).  Does punbb use PHP built-in functions like ob_start(), ob_end_clean(), ob_end_flush(), ob_clean(), ob_flush(), etc...

Also, when should a caching scheme be used?  Dynamic generated ouput that does not change a lot, like menus, headers, and footers?  What steps should I take to apply caching properly?

this may be a better topic for the programming forum, but I was more concerned on how it works within punbb in general.

TIA,

Andrew