A postback URL (also called server-to-server or S2S tracking) is a server-side conversion notification: when a user converts, the advertiser's server pings the affiliate network's URL with conversion data. No cookies or pixels required — the data flows machine-to-machine.
Postback tracking is more reliable than cookie-based tracking and immune to browser privacy restrictions. Most major affiliate networks support postback URLs for sophisticated publishers who need accurate server-side attribution — especially valuable for mobile apps, subscription billing where conversions happen weeks later, and cross-device purchase flows.