What is Collaborative Filtering

We are witnessing the development of new communication technologies (e.g., DTV networks [digital TV], 3G [thirdgeneration] telephony, and DSL [digital subscriber line]) and a rapid growth in the amount of information available. In this scenario, users were supposed to benefit extensively from services delivering news, entertainment, education, commercial functionalities, and so forth. However, the current situation may be better referred to as information overload; as it frequently happens that users are faced with an overwhelming amount of information. A similar situation was noticeable in the 1990s with the exponential growth of the Internet, which made users feel disoriented among the myriad of contents available through their PCs. This gave birth to search engines (e.g., Google and Yahoo) that would retrieve relevant Web pages in response to user-entered queries. These tools proved effective, with millions of people using them to find pieces of information and services. However, the advent of new devices (DTV receivers, mobile phones, media players, etc.) introduces consumption and usage habits that render the search-engine paradigm insufficient. It is no longer realistic to think that users will bother to visit a site, enter queries describing what they want, and select particular contents from among those in a list. The reasons may relate to users adopting a predominantly passive role (e.g., while driving or watching TV), the absence of bidirectional communication (as in broadcasting environments), or users feeling uneasy with the interfaces provided. To tackle these issues, a large body of research is being devoted nowadays to the design and provision of personalized information services, with a new paradigm of recommender systems proactively selecting the contents that match the interests and needs of each individual at any time. This article describes the evolution of these services, followed by an overview of the functionalities available in diverse areas of application and a discussion of open problems. Background The development of personalized information