A Supply Side Platform is a cloud-based software platform used by publishers to monetize their inventory with ad exchanges, ad networks, and DSPs at the same time to sell impressions at their highest value in real time.
Some people often confuse the SSP acronym of Supply Side Platform for Sell Side Platform, but they are very much the same things.
Supply Side Platforms are integrated into a Publishers Ad Server via a few options;
Fully integrated into the Ad Server (Appnexus Supply Side Platform and Appnexus Ad Server are one the same)
Integrated via Header Bidding (ie Prebid.org or similar wrapper)
Installed via Ad Tags typically at the bottom of the Ad Server often referred to as a Daisy Chain)
Supply Side Platforms are uniquely different by company, but they essentially allow a publisher to sell programmatically via PMPs / Deals using a variety of options like Data or Categorization.
Publishers often work with multiple Supply Side Platforms to gain differentiated Demand Sources ie the integrated with Google Adx allows unique demand from Google’s Self Service product, Adwords.
Major Supply Side Platforms include; Appnexus, Rubicon Project, Index Exchange, Pubmatic, Google Adx to name a few.