Google has monopolized the technology used to buy and sell online display ads by restricting or eliminating the choices of its customers—both website publishers and advertisers—the U.S. Justice Department alleged at a federal antitrust trial.

Antitrust enforcers have sought to illustrate how the Alphabet Inc. unit's complex ad ecosystem works and the ways in which the company allegedly manipulated the features of its products and the rules of its auctions to its own benefit. Over the last two weeks at the Virginia trial, they accused Google of abusing its market power in three areas: sell-side tools used by websites, called ad servers; advertising exchanges; and buy-side tools used by advertisers known as ad networks.

Website publishers use an ad server to manage space available for sale. The ad server acts as the brain for the website, keeping track of the minimum bids a publisher is willing to accept, what has been sold and for how much. The Justice Department estimates that Google's ad server controls 87% of the U.S. market and 91% of the market globally.

Ad exchanges control the auctions that match website publishers with advertisers. Google operates the largest exchange, known as AdX, later rebranded as Google Ad Manager. The Justice Department estimates that Google's ad exchange controls 47% of the U.S. market and 56% globally. Other popular ad exchanges include Pubmatic Inc., Index Exchange, and Magnite Inc.

Sophisticated advertisers use software known as a demand-side platform to manage their ads and help determine which ad exchanges to bid on and for how much. Google operates a demand-side platform that can bid on an ad exchange.

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