The only difference is how your ads are being served.
Pop-under - Full page ad will open under the current browser window.
Popup - Full page ad will appear over the current browser window.
Redirects - Full page ad loads in the current browser window itself.
Pop-under - Full page ad will open under the current browser window.
Popup - Full page ad will appear over the current browser window.
Redirects - Full page ad loads in the current browser window itself.