iAd is the advertising network by Apple and is used by iOS apps to display ads inside app. By default, iAd will track your usage so that it can display relevant ads based on your interest. If you do not want this tracking by iAd in place, you have to opt-out.
To opt out of iAd interest-based tracking , visit the link http://oo.apple.com/ on your iOS device. You can opt in again by visiting back the same link. If you have more than one iOS device, you need to opt out from each device. You might still see the same number of ads, but they may be less relevant to you without the tracking info.
iAd by default also track your location so that it can call out ads targeting your location. To disable location tracking, visit Settings > Location Services > System Services and turn off the “Location-Based iAds” switch.