O

Opt-in (or Subscribe):
Opt-In is when a user actively agrees to receive messages. To opt-in or subscribe to an e-mail list is to choose to receive e-mail messages by supplying your e-mail address to a particular company, website or individual thereby giving them permission to e-mail you. The subscriber is usually requested to give personal information such as interests or requested for futher interaction.

Opt-out (or Unsubscribe):
Opt-Out is when the user chooses to no receive messages. Spamming standards require all mailing list messages to carry a link for the recipient to un-subscribe or opt-out from further messages.