Ad Serving is the process of delivering the ads to the user. The delivery happens to the end user´s device and the ad can be displayed by the Ad Server of the publisher or by the Ad Server of the buyer. It will depend of the type of the ad and security process choose by the publisher.
The ads can be load with the normal synchronization of the page, or can be load asynchronous. The asynchronous process means the ad will be loaded after or independently if page loads. The industry standard is now asynchronous as this don´t affect the normal page load.