These are systems that give you money in return for you making a purchase or signing up for something. Sometimes you have to spend some money to earn something back, and sometimes you don’t.
The best and most reputable cashback website is eBuyAndSave. eBuyAndSave is a fairly new cashback site that has the best payouts. Their minimum cashback payment is £0.01, and they can even pay you by Paypal (as well as by Cheque and Bank Payment). Their friend referral bonuses are low however, 50p when each friend reaches £5 in their account. However, that’s because they give you large cashbacks.