The cashback service is provided by http://www.easyfundraising.org and by http://www.easysearch.org.uk.  The easyfundraising site has a comprehensive guide here.

Both these sites make money by receiving an additional bonus payment on top of what we will receive for sending extra traffic to the sites that you are buying from.  

These payments are made because it is more likely that you are going to buy something than someone just browsing normally to a  retailer's site.  

In effect both the club and the sites above are receiving a 'kick-back' payment for you just deciding to use our links instead of browsing directly to the retailers sites!