Newbie questions about updating

[cothrige]

I know this is going to be a very dumb question, but I just can't seem
to get my mind around exactly what is involved and what I should do
regarding this issue.  I understand from reading the handbook that the
ports system is completely separate from the OS itself, and that these
can be upgraded or updated separately.  From what I can see this seems
to most often involve CVSup, and I have been operating under the
assumption that one must run two cvsup operations with two separate
supfiles to update both the core OS and the ports.  Am I understanding
this correctly?

Assuming I am, my main confusion concerns just how these two systems
actually interact and relate to each other, and whether there are any
requirements connecting updating each of them together?  For instance,
I have downloaded the FreeBSD 6.2 install discs and have finished the
basic installation and setup.  Now at some point if I wish to update
the ports does that mean I have to update the OS to a particular
level?  If I don't want to run stable and use "tag=RELENG_6_2" will I
be required to keep the ports as they have installed from the disc?
Is there any connection between how current the ports are and how
current the OS is?

One of the things which caused me to wonder about this was that some
time back I tried FreeBSD out for a while and ran into some oddities
concerning the ports system.  When I first finished setting things up
I could install packages using "pkg_add -r", but noticed that after
updating the ports I could no longer do that.  That struck me as odd,
and because of it I always had a suspicion that I had broken the
system with my out of whack updates (I did not move up to stable at
that time) but I just never could really find out if that were so.

One last newb question is concerning cvsup itself.  In reference to
ports is there a difference, in the end, between this and portsnap?
Do they result in the same ports?  I am sure this is answered
somewhere, but the handbook and other sites seem to take a somewhat
ground-eye view of how to use them but don't dwell much on the
mysteries behind what they do and how they may differ.

Many thanks for any clarification that can be offered to me on these things.

Patrick