update vs. checkout

[Tom Udale]

We have just installed cvs and are finally starting to get a grip on how
to use it.  One conceptual problem so far update vs checkout.  What is
the real difference between the two?  I orginally thought that checkout
was the only way to get newly added directories (and create the local
CVS subdirectories).  Now, from this newsgroup, I see that there is
update -d which seems very much like checkout.  If you always wanted to
update -d, is there any reason not to use checkout?  Is checkout simply
a 'convienient' way of saying update -d (or perhaps with some other
switches) or is there a more fundemental difference?

Thanks,

Tom Udale