If you backup the profile, everything is included: accounts, addressbook, local folders. Local folders are where you might want to save messages that are separate from an account and are local to the computer. Yes, some users use the local folders to offload messages to keep within quota limits, but that is only one possible use.

Hi David, thanks for the reply. Now I understand much better. I have a followup question though. Is a 'profile' based on an email address or on a computer? In other words, I have 3 different email addresses (therefore 3 'accounts') within the one copy of the TBird app on my PC, does this mean I have 3 different profiles, one for each or do I have only 1 profile that encompasses all 3 emails/accounts? Thanks again Chris

A profile is everything involved in one thunderbird setup. all accounts, addressbooks, filters, addresbooks. It is possible to have multiple profiles on one PC, and many do. Tip: in making backup of profile, always make sure that Thunerbir is not running to ensure that you get a full backup.