The recipient fills out a form on a website or otherwise gives permission for the first opt-in. Then the recipient receives a second, automated email asking them to click on a link to confirm that they want to sign up. Reputable companies always use opt-in lead lists when they're sending out email marketing campaigns.
Hope this helps.
If i can add my 2 cents as well its not recommended to use a confirmation after a led has optin . If the lead does not confirm you will lose that lead and it will not get added to your list . Get Response or Aweber have a feature that just requires a single optin ( without confirmation) This is the best for building a list . I guess the true argument is a lead that has to double optin is a stronger lead with more future potential for opens clicks and sales but expect to get 50 percent less leads if you use it .