When someone blocks ad's what really happens?

I've noticed a few websites begging me to turn off adblock, I'm not sure how they detected it though. I think adblock removes the ads entirely from the DOM so perhaps they're just checking if their elements still exist with some JavaScript.

If the Publisher ( Website Owner ) is selling directly ( No Middle Men / Network ) to the Advertiser and hes transparent , he will use a way to track "real" impressions. Ads that aren't shown to the visitors won't count. ( This is only for CPM based ads ) Publishers know that people use adblocker and can easily detect it , it's not uncommon to replace the blocked ads by an internal image / message asking you to disable it on their website.

But yes , if you buy advertising on some websites , you may pay for impressions that never happened.

Feel free to ask more questions on the subject.

From my experience, the ad blocker is actually blocking the ad coming from the incoming server, so no impression is counted.

If you are using something that replaces the ad with another image (Add-Art for example), the underlying ad is likely being served.

The latter is good for the publisher, good for the user, not so good for the ad network or the brand / utility purchasing the ad space.

I'd imagine that you can create a script that would run always that alerts the provider that the user is blocking ads. Then to eliminate false positives you inject some code in your ad script that disables the other script from running. That's probably just one of many possible approaches.