The Internet and the Web have revolutionized access to information. Individuals are depending more and more on the Web to find or publish information, download music and movies, and interact with friends in social networking Web sites. Following a parallel trend, companies go more and more toward Web solutions in their daily activity by using Web services (e.g., agenda) as well as by moving some applications into the cloud (e.g., with Amazon Web services). The growth of this immense information source is witnessed by the number of newly connected people, by the interactions among them facilitated by the social networking platforms, and above all by the huge amount of data covering all aspects of human activity. With the Web, information has moved from data isolated in very protected islands (typically relational databases) to information freely available to any machine or any individual connected to the Internet.

Perhaps the best illustration comes from a typical modern Web user. She has information stored on PCs, a personal laptop, and a professional computer, but also possibly on some server at work, on her smartphone, in an e-book, and so on. Also, she maintains information in personal Web sites or social network Web sites. She may store pictures in Picasa, movies in You Tube, bookmarks in Firefox Sync, and the like. So, even an individual is now facing the management of a complex distributed collection of data.