It is basically a permission based marketing practise which happens mostly on emailers followed with display banners on web. It refers to arrangements between companies to collect user information by providing 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 travel company running a contest may include a checkbox for registrants to accept e-mail for 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. This tool is mostly used to generate leads and these are really convertible compared with other lead generation activity.