Cashback sites are a simple idea. Instead of going directly to a shop, you access a retailer's online store through a link from a cashback website.
You still receive your item directly from the retailer, but you also get some money from the cashback website like commission.
It is often a percentage of the total price you paid or in some cases a fixed amount these are all shown upfront.
The sites have links to retailers of everything from groceries and toiletries, to insurance policies and broadband deals.
Be prepared to wait for your payout however as it has to be approved by the retailer to their affiliate tracking site before it gets passed to the Cashback site who then pay you your share. This can take weeks and sometimes even months for purchases of for example insurance policies or holidays. Straightforward purchases of goods such as beauty products, clothes or food are usually fast paying but the wait will still be 6-12 weeks.
The top 3 sites in the UK are OhMyDosh, TopCashBack and Quidco. They all offer a free account service with the latter 2 providing an enhanced membership where by paying a small annual fee (this can be a one off deduction from your first Cashback amounts of typically around £5 each year) you receive a slightly higher percentage and can take part in Special Offers and Competitions that they run regularly.
In addition they pay bonuses for referring friends and taking up certain offers.
I've had about £220 back so far this year on products I would have bought anyway so it's great getting a discount this way.