Normally on theinternet, if you click on a banner or advert and make a purchase, that website will pay a commision to the original site who had allowed the banner on their site.
What Quidco does is pass back this commision to you the purchaser, instead of keeping it for themselves. Many cashback sites will only give you 50% back of the commision and keep the rest themselves, but Quidco give it all back, minus a £5 a year admin fee, which is only payable if you earn over £5, so if you earn nothing, you pay nothing. £5 is very easy to make though.
Before you make any purchase via the net, check if Quidco has the retailer listed, they have hundreds there, including all the big names. Each retailer will pay varying amounts/percentages in cashback. You still pay the same prices to non-quidco users.
The big and easy ways of making cahsback are clicking through the site for insurance and mobile phone contracts as some of these pay £100.
You can actually make money on bingo.bookie sites where some of them will pay more cashback than they ask you to pay in e.g £20 cashback for a £10 play. You can also make free money by taking out dvd rental trials.