Well this is going to be a short review because TopCashBack is a great website. Like all cashback websites, it gives you cashback for your online purchases at various websites. However, the difference between TopCashBack and the others is that TopCashBack gives 100% (and sometimes more) of its commision to you, without taking an admin fee, so the cashback rates tend to be higher. Quidco also passes on all commision, but it charges an annual admin fee.
Another benefit of TopCashBack is being able to get cashback from other people's transactions, if they don't want to join. You can give out links to websites that go via TopCashBack, so the person can either join or let you have their cashback. For example, TopCashBack are offering £12 for taking up a free trial of Netflix. Instead of linking directly, like I just did, I could link to http://www.topcashback.co.uk/share/moneymatters/netflix. If someone clicks Continue on that page, without joining or logging into TopCashBack, I would get the £12 cashback. If they joined TopCashBack, I would get a referral fee, which varies. You don't need a website to do this, you can just email the URL to friends.
There is a full guide to cashback websites on moneysavingexpert.com, so I won't review them all, but I've tried a few and, in my experience, TopCashBack is the best.