All these sites are based on conversion, i.e. your hit turns into a sale with their cookie in your machine and they then get the commission on the sale, which they then pass on to you - i.e. quidco.
Many of these websites are part of what's called affiliate groups, ecommerce sites use them to punt out deals / voucher codes / discounts of some description and thus get more hits and sales.
In reality this is nothing more than getting discount in store or getting a coupon through the post - however, it's such an easy way to save cash and with quidco it feels like you're earning money. Which, you can do though various methods make some profit.