The amount of information available online is increasing exponentially. While this information is a valuable resource, its sheer volume limits its value. Many research projects and companies are exploring the use of personalized applications that manage this deluge by tailoring the information presented to individual users. These applications all need to gather, and exploit, some information about individuals in order to be effective. This area is broadly called user profiling. This chapter

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... eys some of the most popular techniques for collecting information about users, representing, and building user profiles. In particular, explicit information techniques are contrasted with implicitly collected user information using browser caches, proxy servers, browser agents, desktop agents, and search logs. We discuss in detail user profiles represented as weighted keywords, semantic networks, and weighted concepts. We review how each of these profiles is constructed and give examples of projects that employ each of these techniques. Finally, a brief discussion of the importance of privacy protection in profiling is presented. general, systems that collect implicit information place little or no burden on the user are more likely to be used and, in practice, perform as well or better than those that require specific software to be installed and/or explicit feedback to be collected. Methods for user identification Although accurate user identification is not a critical issue for systems that construct profiles representing groups of users, it is a crucial ability for any system that constructs profiles that represent individual users. There are five basic approaches to user identification: software agents, logins, enhanced proxy servers, cookies, and session ids. Because they are transparent to the user, and provide cross-session tracking, cookies are widely used and effective. Of these techniques, cookies are the least invasive, requiring no actions on the parts of users. Therefore, these are the easiest and most widely employed. Better accuracy and consistency can be obtained with a login-based system to track users across sessions and between computers, if users can be convinced to register with the system and login each time they visit. A good compromise is to use cookies for current sessions and provide optional logins for users who choose to register with a site. Web usage mining can also be used to identify users, and these approaches are covered in more detail in Chapter 3 of this book [59] . Many companies rely on data aggregators, such as Acxiom [1], to provide demographic data about customers. This information actually turns out to be more accurate than surveys of customers themselves. Usually, all that is required to get full demographic data is a credit card number or the combination of name and zipcode, information that is often collected during purchase or registration. The first three techniques are more accurate, but they also require the active participation of the user. Software agents are small programs that reside on the user's computer, collecting their information and sharing this with a server via some protocol. This approach is the most reliable because there is more control over the implementation of the application and the protocol used for identification. However, it requires user-participation in order to install the desktop software. The next most reliable method is based on logins. Because the users identify themselves during login, the identification is generally accurate, and the user can use the same profile from a variety of physical locations. On the other hand, the user must create an account via a registration process, and login and logout each time they visit the site, placing a burden on the user. Enhanced proxy servers can also provide reasonably accurate user identification. However, they have several drawbacks. They require that the user register their computer with a proxy server. Thus, they are generally able to identify users connecting from only one location, unless users bother to register all of the computers they use with the same proxy server. The final two techniques covered, cookies and session ids, are less invasive methods. The first time that a browser client connects to the system, a new userid is created. This id is stored in a cookie on the user's computer. When they revisit the same site from the same computer, the same userid is used. This places no burden on the user at all. However, if the user uses more than one computer, each location will have a separate cookie, and thus a separate user profile. Also, if the computer is used