Opt-in/Opt-out
Opt-in: to give permission to receive information or advertising over the Web.
Double Op-In: when a person requesting information must also respond to an e-mail confirmation to continue to receive future e-mails.
Opt-out: to request to be removed from an e-mail list. With most spam (sent by bots) opting out usually does not work and sometimes results on getting your name on even more lists.