Let's talk about recommender systems.

Motivations to talk about recommender systems

It's an important application of machine learning
Many groups in Silicon Valley now is trying to build better recommender systems. There are many websites or systems in Amazon, Netflix, eBay, iTunes Genius try to recommend new products to use. Amazon tries to recommend books to you. Netflix tries to recommend movies to you and so on. These systems are responsible for a substantial fraction of the revenue of these companies. So the improvement in performance of a recommender system can have a substantial and immediate impact on the bottom line of many of these companies.