"People who bought this also bought these."
When you shop online, the site recommends some items to you based on your shopping history and the products that you looked at and probably liked. The system that generates these recommendations is known as a recommendation engine. There are several types of state-of-the-art algorithms used to create a recommendation engine. In this chapter, a couple of such types of algorithms will be discussed, namely "collaborative filtering" and "content-based filtering".