Recommender systems were often used to show the candidates such as books, movies and music for certain user. If you bought something at online shop, you might get a spam email listing something the shop recommended and there must be a recommender systems for you. You might get confused by such system: how does they know what you would buy?
If you know basic concepts about machine learning, you might say: surely there would be a supervised learning algorithm for this problem. Yes, when you have huge data, you might train your algorithm with rating as your output and items as your …