VAST (Video Ad Serving Template) is the IAB standard an ad server uses to describe a video ad to a player or ad stitcher: where to fetch the creative, its duration, and the tracking/beacon URLs to fire at events like start, quartiles, and complete. It is the lingua franca between the demand side (ad servers, exchanges) and the playback side.
VAST handles a single ad (or a wrapper that redirects to another ad server). Its companion VMAP (Video Multiple Ad Playlist) describes the ad schedule — which breaks exist in a piece of content and what goes in each (pre-roll, mid-rolls, post-roll). Together they let a player or SSAI stitcher request, schedule, and report on ads in a vendor-neutral way.
Both SSAI and CSAI rely on VAST/VMAP: in CSAI the player's ad SDK consumes them directly; in SSAI the stitcher consumes them server-side and weaves the resulting creatives into the stream. Correct VAST tracking is what makes ad impressions countable and billable.
