An opt-in list is one that you have to specifically request to be on - like signing up for a newsletter. If you opt-in, it's not really spam, because you requested it. There is usually a fairly simple way to get off the list in this case. (This is not a technique used much by spammers.)
An opt-out list is one where you are signed up for the list without your knowledge and you are responsible for removing yourself. Opt-out is usually a sign of bad intentions! Many people feel that any legislation which legalizes opt-out marketing is (almost) worthless.