A demand-side platform is a company that provides technology for media buyers to purchase ad placements, typically via bids in ad exchanges' real-time auctions. Simply, a DSP is a computer-based platform that automates media buying across multiple sources.
In the world of digital, the term "buying" has been replaced by "demand". Its inverse is "supply" so publishers and media owners are now considered "supply side." In a DSP, data are treated like media in that it is layered across the buy and becomes just another part of the cost.
DSPs do not own, purchase, represent or resell inventory from publishers, but connect to an ad exchange or a Supply Side Platforms (SSPs) so that publishers can sell inventory themselves. An SSP is a DSP, but from the publisher or content producer's perspective, so it is a technology platform that automates the selling of ad impressions for publishers and media owners.
An SSP typically might include capabilities like Real Time Bidding (the dynamic process of buying and selling impressions instantaneously in which the highest bidder "wins" the right to place a display ad while the audience attributes are fixed in real time), yield management and the appending of audience attributes.
Yield management, or yield optimization, from a publisher perspective, is the process of figuring out how much your impressions are worth and how can you manage the flow of inventory to make the most money? SSPs look at each impression available on a web publisher's site and then match the impression with an available ad from an ad network or exchange. Typically this has been done as an assessment of remnant inventory, but could be applied to all inventory. Certain DSPs can also be channel specific, just for video or mobile ads.
Companies in the DSP space include Google's DoubleClick Bid Manager and Invite Media, DataXU, MediaMath, X+1 and Turn. Companies providing SSP technology include AOL, AppNexus, Google, OpenX, PubMatic, The Rubicon Project and Right Media.