In case of the Netflix competition, the data there relies on the explicit feedback given by the users. The users went to the website and explicitly told them how much they like the movie by giving it a rating from 1 to 5.
Typically it is quite difficult to make users do that. However, just by visiting the website and interacting with it, the users already generated a lot of useful information, which can be used to infer their interests. All the clicks, page visits, and past purchases tell us about the preferences of the user. This kind of data is called implicit - the users do not explicitly tell us what they like, but instead, they indirectly convey this information to us by using the system. By collecting this ...
