I find it easiest to think in terms of mailing lists when it comes to email marketing. Email marketing is nothing else but mailing lists in the end. The mailing list of an opt-in marketing campaign starts empty. The only way for an email address to get onto the list is by its owner explicitly requesting it.
The request must always be for a specific list. If a customer submits a form that says "Yes, send me a copy of your current catalogue" you can add them to the list of customers who will receive a copy of the current catalogue, use the list one time and then throw it away. To continue sending them your latest catalogue, you need the explicit permission to do so (along the lines of "Yes, keep me up to date by subscribing me to your list of catalogue recipients").
"Permission-based" means essentially the same. Marketing material is sent only with permission, and often personalized.
