Real-time bidding is an auction that happens in about a hundred milliseconds. When a user opens a page, a bid request fires. Multiple advertisers evaluate the impression, decide whether they want it, and submit prices. The highest bid wins and the ad renders before the page finishes loading.

Nothing about this is visible to the user. It is plumbing, but the plumbing determines what ads appear everywhere online. Speed and automation are the whole point.

How a bid request flows

1. A user loads a page with ad space
2. The publisher sends a bid request to exchanges
3. Advertisers evaluate and submit bids
4. The winner's ad is served and the impression logged

RTB is a subset of programmatic buying. Not all programmatic transactions are auction-based. Some use fixed-price deals negotiated in advance.