In e-mail terminology, co-registration refers to arrangements between companies to collect user information. Usually this would be a separate check-box on a Web signup form where the user can opt-in to receive messages from a third party. For example, a magazine running a contest may include a checkbox for registrants to accept e-mail future announcements from them, and also from the prize sponsor of the contest. Co-registration usually adds the subscriber, provided they check the boxes to give permission, to two or more opt-in mailing lists.
See "How Affiliate Marketing Works" in the "Did You Know..." section of Webopedia.
Also see Refer-It.com, a directory of referral & affiliate programs for e-commerce, featuring information on over 6,500 affiliate programs.