File sharing is the practice of sharing digitally stored information. This involves distributing and providing access to computer programs, electronic books, documents, pictures and multi-media (audio and videos). The main use of file sharing technologies today is to provide access to movies, music, computer programs and games. But it is also used for many other purposes such as sharing academic papers. File sharing is done by three distinct methods. The information can be physically transfered on a removable media, it can be distributed from a central file server on a computer network or it can be shared on a distributed peer-to-peer network.
Types of file sharing
Anonymous file sharing networks are usually based on peer-to-peer technology. By adding encryption and using intermediaries for data transfer they become safer and peers can not identify who they are receiving or sending data from. Some of these network also provide more control over who you share with and could be described as friend-to-friend networks. Anonymous network are still rather small, some of the more popular alternatives are Freenet and OneSwarm. One can also use regular peer-to-peer protocols with anonymous overlays such as I2P or Tor.
Peer-to-peer or P2P networks are based on every member of the network sharing content with everyone else. While hosting of content is distributed in the network many P2P technologies still depends on a central server to co-ordinate the peers. The Direct Connect protocol depends on central nodes called hubs for peers to find each-other and BitTorrent depends on trackers. There are technologies who are less dependent on a central node to find other peers such as Gnutella but some co-ordinated way of finding the first other peer in the network to connect to is still necessary in all P2P networks.
File hosting services
With a file hosting service all the information is stored on one server and every client must connect to it to download the information. This is a centralized method of sharing information to a network and the most common protocols are FTP and HTTP.
Removable media file sharing is when the information is stored on a external device such as a USB flash memory and transfered to another machine. Removable media is the oldest type of file sharing starting with punch cards for machine instructions in the 18th century.
Digital information has been around and been shared since the invention of machine instruction punch cards in 1725. With the launch of compact audio cassette in the 1960s file sharing become a phenomenon to speak of. Music could easily be copied and distributed on these cassettes. The VCR made this possible for movies as well, but the loss of quality in home made copies made distribution less popular with video.
With the development of the internet it became possible to share information without physically moving it and file sharing activity moved to a global scale. The first systems for file sharing on the internet where bulletin boards. In 1979 Usenet was formed, in 2006 Usenet still remained the main source of file sharing traffic on the internet. Usenet is a semi-distributed network built around newsgroups.
The next major step was the launch of Napster in 1999  Napster was a P2P network but relied on a single server to index and control th entire network which made it vulnerable to legal action and it was shut down following a lawsuit in 2001. Napster was replaced by the less centralized P2P networks such as the FastTrack network used by the Kazaa client(2001) and the other networks Gnutella(2000) and Direct Connect(1999). Freenet was released in 2000 as the first anonymous network.
In 2002 and 2003 the BitTorrent protocol emerged with the founding of several tracker sites including The Pirate Bay. As lawsuits began plaguing Kazaa and it's users, BitTorrent became more popular and many more public and private trackers emerged. By 2009 internet is dominated by BitTorrent traffic, in several regions over half of the internet traffic is in the BitTorrent protocol
Uses of file sharing
Peer-to-peer networks are mainly used to illegally distribute copyrighted content such as movies and music. While legal distribution is more often done with file hosting services. There are many services legally streaming music and video. File sharing is also used in academia to distribute academic journals and share academic libraries.
- From Usenet to CoWebs: interacting with social information spaces, Christopher Lueg, Danyel Fisher, Springer (2003), ISBN 1852335327, ISBN 9781852335328
-  RFC 3877, Network News Transfer Protocol (NNTP), C. Feather, The Internet Society (October 2006)
- Kenneth P. Birman (2005). Reliable distributed systems, p.532. ISBN 0387215093.
- Internet Study 2008/2009 ipoque 2009. Hendrik Schulze, Klaus Mochalskia