We also recommend placing a link in the header of your email as well. The option to do this is in Step 1 of the email builder.
When a subscriber clicks the link and unsubscribes, he/she is automatically removed from your contact list(s) and placed on your Master Unsubscribe List.
A person who has unsubscribed is still able to subscribe again, but there is a certain process that needs to be followed in order for this to take place. You will find instructions here.