When shopping online, click through to a shop via a ‘cashback’ site and it'll give you a small proportion (usually around 2% but up to 10%) of what you spend back in cash.
These are advertising sites, paid for by providing leads to retailers; to encourage traffic, they split some of their revenue with you in the form of cashback.
How much? Up to £1,000 a year if you’re a serious online buyer.