One of the most common applications of machine learning systems is to recommend things to users that they'll be interested in. Have you noticed how Spotify and Pandora recommend a certain kind of music, or particular songs or radio stations? You may have observed Netflix recommending movies for you, as well, as in the following screenshot:
How about Amazon recommending books based on the book that you are currently browsing, as in the following screenshot:
Such systems are known as recommendation systems.
A recommendation system is one that learns about what items might be of interest to a user, and then recommends those items for buying, renting, listening, watching, and so on. Recommendation systems are broadly classified into two categories: content-based filtering and collaborative filtering.