During browsing, the user's attention (and their money) is up for grabs. By showing the user something compelling, a web site maximizes the likelihood of a transaction. So if a web site can increase the chances of giving users good recommendations, it makes more money. Obviously this is a difficult problem, but the incentive to solve it is very big. The main approaches fall into the following categories: Personalized recommendation - recommend things based on the individual's past behavior Social recommendation - recommend things based on the past behavior of similar users Item recommendation - recommend things based on the thing itself A combination of the three approaches above