A URL with tags to measure the effectiveness of a campaign and to record which affiliate partner sales are coming from, to enable that partner to be awarded the commission.
Example: An tracked affiliate link will typically look like the below (it varies by network or tracking partner) - it will often clearly show the network used and the affiliate ID/name.
https://www.awin1.com/cread.php?awinmid=11193&awinaffid=1084731&ued=https/www.karl.com/gb-en
References: Affiliate tracking