Well - briefly:
If someone subscribes to an x-cart newsletter from an x-cart form or the register page, then the email address is instantly mapped across into the mailchimp account with first name and last name.
The email is still subscribed to the newsletter within x-cart.
Unsubscribe works the opposite way - deleted from both mailchimp and x-cart.
You can also use the Mailchimp 360 commerce - which enables you to send a email blast out with Mailchimp - and track the results through your x-cart store... I haven't done this yet - but can't wait.
To set that up for this module - you just check a box
edit - it supports multiple lists