First

Publisher (Media Company, Affiliate, Seller, Supply partner) are considered first party and use the following technology:

• First-Party ad server
• Ad - Sell-Side Platform (SSP)

Second

• Ad - Ad Network (Vendor)
• Ad - Ad Exchange

Third

Ad - Advertiser and Ads - Agency are considered third party and use the following technology:

• First-Party ad server
• Ads - DSP (Demand-side platform)