The below paragraph is taken from MoneySavingExpert
Cashback sites take advantage of the way commercial payments from one website to another work. They use affiliate links, which allow the retailer to track where the traffic is coming from and then pay the cashback sites for the lead.
This is a common system, used by sites that send people through from comparison results, unique content or using links on advertising promotions. Cashback sites simply drive traffic by giving their users some of the money they're paid.
The amount of money depends on what's spent on what as well as the commercial deal, so can vary widely. The cashback site may earn its money per click, transaction, application, or accepted applicant.
The technology's simple. Ready-made paying links are available from 'links warehouses'. Big cashback sites also have direct relationships with companies, which means they can offer a wider range of providers, earn more and negotiate their own exclusive deals.