Real-time bidding (RTB) is a new method of selling and buying online display advertising in real time one ad impression at a time. Also sometimes referred to as programmatic buying/selling. Instead of bulking buying and inventory-centric buying, RTB mimics stock exchanges and utilises computer algorithms to automatically buy and sell ads in real-time. An empirical analysis  suggests that RTB encourages the use of per impression context and targets the ads to specific cookies, primarily based upon demographic and behavioral data, and hence dramatically increases the effectiveness of display advertising.
There are 3 groups of entities involved, just like in financial markets:
- buyers and systems that use Demand Side Platforms (DSP), i.e. Trading Desks.
- 24/7 Trader (MIG/24/7 Real Media)
- AcuityAds  offers a full serve and self serve platform for programmatic real time media buying.
- ADMAXIM  enables brands to navigate the complexity of mobile media and capture the full potential of engaging with consumers on-the-go.
- ADREACH  by Adstream is a leading edge, end-to-end digital advertising and marketing platform that uses the most advanced learning algorithm technology to deliver significantly improved returns for clients, agencies, media owners and developers across mobile and online display, text, video and live streaming video ads.
- AdServingSolutions.com (ad serving RTB DSP)
- Adform 
- ADMIZED 
- Avazu DSP 
- BidTheatre 
- BidMath, is a Self-serve RTB DSP, exclusively for mobile and largest in Canada
- ChoiceStream 
- Collective Roll, a leading results-driven managed RTB provider
- Delta Projects 
- DoubleClick Bid Manager (formerly Invite Media)
- EQ Works
- Fractional Media
- Go2mobi  is a company at the forefront of real-time bidding, driving over 2 million clicks every day.
- Infectious Media
- PocketMath  is a mobile DSP for buying in-app and mobile web inventory for smartphones and tablets, worldwide - all via RTB.
- ReTargeter (retargeting DSP)
- RTB House
- The Trade Desk
- Tradelab Independent Trading Desk based in Paris and Singapour.
- Wave Digital Media Real-Time Bidding, Evolved.
- WAY2TRAFFIC / W2T DSP
- Zebestof International Trading Desk
- sellers who use Supply Side Platforms (SSP)
- online real-time ad exchanges
A certain user visits a website with one displayed advertising. A call is then made by the exchange server (database web server) supporting Real Time Bidding (RTB) to the Demand Side Platforms (DSP) or Ad Networks (Ad Exchange) to determine which advertiser gets to serve the ad. Each user has an associated set of attributes (cookies), which is transferred from exchange server to the DSP and determines whether the user has the desired attributes (cookies) that the advertiser wants to target. Based on the perceived value of this user, a bid is placed on this ad impression by relevant advertisers and the highest bidding advertiser gets the placement
Let's explain with a simple real time example: a user heads to a page on a website (the "publisher"), causing it to start loading. In the same instant the website ("publisher") sends out a “bid request” to thousands of potential advertisers saying, “We’ve got this user who is 30, Indian, male and based in New Jersey, US, and recently searched for return air tickets to Delhi, opening a page on our site. How much are you willing to bid for being the only ad on this page?” Within about 100 milliseconds the publisher receives bids from different advertisers, which in turn analyzes the group to determine both the highest bidder and the brands being advertised. The winner is alerted by the publisher and allowed to place its ad on the page. The remarkable thing about this entire process is how fast and how often it takes place. The entire series of to-and-from communication between publisher and advertisers takes place in 300-500 milliseconds, causing no visible delay to the user. This process is repeated for every ad slot on a page.
What happens in the background
From the technical stand point the following typical transactions take place.
- A user's browser requests a webpage of a site that uses real time bidding
- The user's browser receives the HTML for the webpage. Embedded in this HTML is a URL tag for an ad request
- The user's browser makes a call out to the ad server
- The ad server calls out the SSP that this website is using. Sometimes SSP and the ad server are integrated such as 24/7 Real Media OpenAdstream or Google's DFP & AdX systems.
- SSP makes a call out to one or more ad exchanges
- An ad exchange makes multiple call outs (bid requests) to several DSPs trying to get the best price
- DSPs evaluate the bid request and decide how much they want to bid and respond with bids to the exchange
- The exchange selects the highest bid with a URL of the winning ad and returns that to the SSP
- SSP returns the winning bid to the ad server
- The ad server finally responds with an ad to the browser of the Internet user which started all these transactions
For a schematic you can refer to 
NB: this method describes a 'back end' ad server/SSP integration where the ad server talks directly to the SSP platform, other methods include a client-side method where the ad server sends an SSP HTML tag back to the initial request which in turn calls the SSP directly, the SSP runs the mediation/auction process as described above and returns the final ad to be delivered to the user's browser.
- Ad blocking applications on web browsers (like Adblock Plus) circumvent the delivery of advertising (and in doing so, deprive the site publisher of important advertiser revenue).
- Buyers are able to acquire impressions they want at a price that matches their goals and objectives. For buyers, RTB offers the possibility of buying only the impressions they need at a price they are willing to pay by utilizing proprietary optimization algorithms of DSP systems and data from various 3rd party providers to fuel their algorithms.
- Sellers use SSP with supply side algorithms to get the maximum yield out of ad inventory which is available to them to sell. The supply side algorithms are focused on optimizing yield by manipulating floor prices.
- RTB involves a real time auction for each ad impression in which buyers and sellers come to an agreement on the value of each ad impression and concluding the transaction.
- The basic premise of RTB is that ad impressions are not commodities to be sold in bulk, and that each ad impression is unique - the time of day, the place or website, the audience, the propensity of that audience to make a purchasing transaction at that particular time and place and given their transaction history.
- Auctions take place at a blink of an eye. A typical exchange imposes one tenth of a second for the bids to return.
- Volumes of bid requests and bids are also staggering. Google AdEx alone serves up 100 thousand bid requests a second.
- Real-time bidding
- Programmatic Marketing
- Ad exchange
- Advertising network
- Online advertising
- Demand-side platform
- Sell Side Platform
- The Arrival of Real-Time Bidding, Google White Paper
- What is Real-Time Bidding (RTB)? by Alexia Nielsen
- Real-time Bidding, blog by Adfusion
- How do ad exchanges and real-time bidding platforms work? by Miten Sampat
- RTB Exchanges - 4 Ways for Ad Ops Teams to Adapt by Rob Beeler
- Real-Time Bidding: The Ad Exchange Leaps, an article in Adexchanger
- Define It - What Is Real-Time Bidding?
- "Real-time Bidding for Online Advertising: Measurement and Analysis".
- ZAP resource center
- "Acuity Ads - We Are Real Time Bidding"
- "Real-Time Bidding Technology"
- "Ad Exchange RTB platform"
- "European DSP"
- "DSP & Trading Desk"
- "DSP for Europe and Emerging Markets"
- "DSP for Swedish and European markets"
- "Self-serve RTB DSP, exclusively for Mobile"
- Bonadza, a Major Self Serving DSP
- "Full service optimization of RTB campaigns"
- "RTB DSP
- "Real-time Audience & Media Platform
- "Programmatic Marketing
- "A smarter and different DSP"
- A smarter approach to powerful, precise display buying
- "EQ Works
- "Fractional Media". Fractional Media. Retrieved 2013-10-04.
- "Mobile Ad Network | Mobile RTB". Go2mobi. Retrieved 2013-10-04.
- "Performance Display Platform"
- Infectious Media - a UK Trading Desk
- Mobile Advertising optimization
- "Mobile ad optmization based on results"
- A Global Ad Exchange
- "Holistic RTB buying"
- "Home". PocketMath. Retrieved 2013-10-04.
- "Ad Retargeting RTB platform"
- RTB Self Trading Desk
- "Real-Time Bidding And Retargeting Technology"
- RTB for ROI optimization
- The Trade Desk
- "DSP". TLVMedia. Retrieved 2013-10-04.
- "Tradelab, spécialiste des campagnes display en Real-Time Bidding". Tradelab.fr. Retrieved 2013-10-04.
- "Real-Time Bidding | Mobile RTB, Video RTB & Display RTB". Wave Digital Media. Retrieved 2013-10-04.
- "Affiliate and ad management system". Way2traffic. Retrieved 2013-10-04.
- Zebestof - a new generation Trading Desk
- SSPs, aka Supply Side Platforms
- Explainer: Supply-Side Platform, Digiday
- "Marketplace by ADTECH"
- A Global Ad Exchange
- Real Time Bidding from the Publisher perspective, white paper, Pubmatic
- "Ripple - Smart Media for Smart Screens"
- "Adloop | Sell Your Premium Ads Programmatic". Adloop.de. Retrieved 2013-10-04.
- "Private Ad Exchange". ADMETA. Retrieved 2013-10-04.
- Google Buying Supply Side Platform Admeld For $400 Million, article in paidContent.org
- Adiquity - Asia's First Real Time Bidding Exchange
- 24/7 Adding RTB To Open AdStream
- Ad optimization technology
- "Global Ad-Exchange"
- A Global Ad Exchange with Real-Time Bidding
- Leverage your own technology and data for efficient media buying
- "The World Leader in Real-Time Advertising Technology"
- "The Platform for Premium Digital Advertising"
- Kevin Reynolds. "Are Ad Exchanges and Real Time Bidding The Next Big Thing?". Advertisingperspectives.com. Retrieved 2013-10-04.
- "Diagramming the SSP, DSP, and RTB Redirect Path". Ad Ops Insider. 2010-12-15. Retrieved 2013-10-04.
- The mission of the OpenRTB project
- OpenRTB: Open Standards for Real-Time Bidding Communication for the Online Advertising Industry
- OpenRTB Momentum Builds