A recommendation system attempts to predict items a user may be interested in, like movies, music or books, to help people find interesting information. For ecommerce, recommendation systems can suggest additional or more expensive items. Common approaches include collaborative filtering based on user preferences, content-based filtering using item descriptions, and hybrid methods. Effective recommendation systems face challenges like data sparsity, scalability, and "shilling" attacks. Major companies report significant sales increases from recommendation features, showing their business value.