How does paid to sign up works?

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Can someone explain to me how does it works? I mean when we sign up for an offer in a site, then they say that I have to submit a the confirmation mail which is sent from the site I have signed up, who confirms this email? The advertiser? or the site owner? If its something else please explain as I have to implement paid to sign-up system in my site. Thanx In Advance. (I am talking about paid to sign-up offers in ptc and ptr sites)