Unsupervised learning is about analyzing the data and discovering hidden structures in unlabeled data. As no notion of the right labels is given, there is also no error measure to evaluate a learned model; however, unsupervised learning is an extremely powerful tool. Have you ever wondered how Amazon can predict what books you'll like? Or how Netflix knows what you want to watch before you do? The answer can be found in unsupervised learning. We will look at a similar example of unsupervised learning in the following section.