The full form of ADX is DoubleClick Ad Exchange.

Google’s DoubleClick Ad Exchange (AdX) is the big one. Verizon-owned AOL operates exchange-SSP hybrids for display and video called ONE by AOL: Display and ONE by AOL: Video. The Rubicon Project, AppNexus, PubMatic and OpenX are among some of the biggest independent exchanges.

Authorized Buyers (formerly DoubleClick Ad Exchange) connects ad networks, agencies, and demand-side platforms with real-time inventory.

On an ad exchange, thousands of publishers can make unsold ad space inventory available for advertisers to bid on automatically — as opposed to traditional one-on-one sales negotiations. The buying and selling happens programmatically, which is mostly a fancy term for automatically (Check out our primer on programmatic), between software platforms.

Advertisers and agencies typically connect to an ad exchange using what is called a demand side platform (DSP) — MediaMath, Turn, DataXu and AppNexus are some of the big DSPs. Ad networks also often buy impressions on ad exchanges, then mark them up and resell the impressions to buyers on the networks. Publishers typically make impressions available on an exchange through a supply-side platform (SSP). We’ll get to more on SSPs in a bit.