In this chapter, we did a quick survey of an e-commerce site, its variations, and the importance of having recommender systems. We can now say that a customer will only buy products that are visible. A product that is hiding down below the pages and links is not likely to be bought by a customer. This is where a recommender system helps an e-commerce site, that is, with such a huge number of items to search through, a customer is now able to narrow his/her search with ease. In the next chapter, we will learn about different machine learning algorithms that will help us pick and evaluate the techniques for building a recommender system.