This is the status bar, showing you requests being made to the web server.

Web servers host the content of each website, like articles, videos, or images.

Some servers also host advertising content. These are called ad servers.

In order to show an ad, a website sends a request to an ad server.

After receiving the request, the ad server looks through all available display banners

to pick the one that's most relevant one for you.

For example, if an advertiser is promoting different products for different regions,

the ad server will find the best one to show you based on your current location.

Once the ad has been selected, it will be returned to the page that you're viewing

"After you've seen the ad, the ad server counts an impression,

tracking that the ad has been served. "

Ad servers also track clicks, so advertisers can see how people are responding to their

different ads.

Because the ads are controlled by the ad server, once an advertiser sees that one ad is more

popular than others, they can adjust the settings in the ad server so the winining ad serves

more often.

This process -- from the ad server receiving the request, selecting the best ad, and then

counting an impression - happens with each ad that's served. It only takes a fraction

of a second to match an ad with each request.

This dialogue between the web page, web server, and ad server is called third-party ad serving.

Because it allows for them to report on impressions and clicks - and also change their ads on

the fly - third-party ad serving is a common way for advertisers and ad agencies to manage

online display campaigns.

Now that you know what's happening behind the scenes, keep an eye on the lower corner

of your browser the next time you're surfing the web.