Are you in the UK? I'm not sure where else it's available. Basically It gives cashback deals and discounts for certain products or services. It also give smaller cashback deals for certain stores (2%-5%) but if you shop with those stores a lot it soon mounts up. Mobile contract deals are ususally very good, giving appox £100 cashback, Sky subscription is also pretty good as are insurance deals. it can either pay the cashback into your account every week or you can "savei it up" till a significant amount is reached.
Join here, no cost to join, it's a no brainer, if you don't like it then don't use it.
http://www.quidco.com/