A DSP is a demand-side platform, an online system through which advertisers can automate their purchases of online ad space. An SSP, or a supply side platform, is how publishers who supply ad space connect with buyers and automate sales of their ad inventory. An example of an SSP is Amazon Publisher Services (APS), which connects third-party web publishers with advertisers that use the Amazon DSP. This is how the Amazon DSP is able to supply advertisers with ad inventory off Amazon and Amazon properties, to third-party websites.