I've just learnt about this, so may be a bit behind the times, but its basically a cashback website, you just sign up for an account (
http://www.topcashback.co.uk/ref/new_registration" target="_blank ) and once you have done that, search in the search box for the name of the website (eg eBay, Amazon) you are planning to buy from, itll take you to the online shop, and you just buy like normal, except because you clicked though the Topcashback website first, itll track your purchase, and once the purchase has gone through you get a percentage cashback into your account which you can withdraw either to your bank account or Paypal account.
I have only just started using it, but bought something on eBay the other day for £15, and got 53p cashback, its not huge but if you are buying expensive items, or lots of them, or even just over the space of a few months, it will build up a nice pot of money you wouldn't have otherwise. It has a massive amount of shops, Amazon, eBay, Argos, Asda, etc etc