Affiliate marketing is a marketing arrangement in which affiliates receive a commission for each visit, signup or sale they generate for a merchant.
In Odoo, Link tracker can be used for affiliate marketing purposes. The link tracker is only visible in developer mode, Enable Developer mode from General Settings.

Open the Link Tracker app and create a new Tracked Link

Paste the Target URL for the page that will be redirected from the link.

Setup Campaign, Medium and Source, and save the record. A tracked URL will be generated.

The tracked URL can be then given to a partner, and they can use it to send visitors to the target URL.

When an order is placed from the visitor using this link, the information of campaign, medium and source will be captured on the Sales Order.

Orders can then be filtered, and grouped by different campaign, medium, source to then correctly get the total value of the order and payout a commission to the Affiliate.

In addition, this can be used with Commissions in Odoo to add a referrer to the order in order to payout the person as invoices for sales orders are paid for by the customer.

Please review the following post regarding setting up commissions in Odoo.

https://www.odoo.com/forum/help-1/how-do-i-setup-commissions-in-odoo-226633

The following Automated Rule can be used in order to set the Referrer automatically.

Note: One automated rule per Affiliate has to be created.