Brief overview exchange outlook connection please

Hi, I am wanting a brief overview on how outlook 2003 and Exchange server
2003 synch new mail messages. Does outlook query the server for the new
messages every so often or when Outlook is connected in online mode, am I
simply viewing my mailbox on the exchange server? I know that the mail is
left on the server and not downloaded per say, but we have a strange issue
where a user can see mail if viewing through OWA, but delayed when trying to
view through Outlook, and the user is not using a .pst file, they are storing
their mail on the server side.

Thanks in advance
Mike