Recommendation engines aim to show user items of interest. What makes them different from search engines is that the relevant content usually appears on a website without requesting it and users don't have to build queries as recommendation engines observe user's actions and construct query for users without their knowledge.
Arguably, the most well-known example of recommendation engine is www.amazon.com, providing personalized recommendation in a number of ways. The following image shows an example of Customers Who Bought This Item Also Bought. As we will see later, this is an example of collaborative item-based recommendation, where items similar to a particular item are recommended:
In this section, we will introduce key concepts related to understanding and building recommendation engines.