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
Group items tagged 
