An incentive is a motivator for users to perform an action. Incentives can be, for example, financial, moral (e.g. do something good), instinctive (e.g. curiosity).
Besides forming a clear motivation for a single action, incentives can also be viewed in a more wide context of economics. Arguably, most of consuming behaviour is driven by different kind of incentives and the consumers' desire for them. (http://en.wikipedia.org/wiki/Incentive , retrieved 2013-02-05)