VAST Tag aka Video Ad Serving Template is a script for the video ad that supports only in-stream video ad formats that gives consistent instructions to a video player on how to handle an ad. It tells the video player what the ad should do, how it should show up in the player, how long it should display, whether or not it is skippable, where to find the ad (i.e. the ad server), and what the click-thru URL should be. There are multiple variations of this type of tag such as VAST2 & VAST3 which have update security and advanced ad serving and video playback capabilities.