A snapshot is taken first, so this can be undone from Backup. But the CSV cannot give back what is not in it: groups will be emptied, purchases recorded against contacts will be gone, and consent flags will be unset.
Everything — contacts, groups and templates, with ids and timestamps. This is the only export that can rebuild the database; the CSV cannot.
Taken automatically every night, before every CSV import, and before any restore. The most recent 30 are kept.
Anyone with the consent box ticked goes to Mailchimp automatically when they are saved, whether they signed up on the website or were typed in here. Contacts without consent are never sent. This is only for catching up after a big import, or when Mailchimp was down.