Opt-in e-mail is e-mail containing information or advertising that users explicitly request (opt) to receive. Typically, a Web site invites its visitors to fill out forms identifying subject or product categories that interest them and about which they are willing to receive e-mail from anyone who might send it. The Web site sells the names (with explicit or implicit permission from their visitors) to a company that specializes in collecting mailing lists that represent different interests. Whenever the mailing list company sells its lists to advertisers, the Web site is paid a small amount for each name that it generated for the list. You can sometimes identify opt-in e-mail because it starts with a statement that tells you that you have previously agreed to receive such messages.