A torrent is a file shared across a peer-to-peer (P2P) network of private computers, versus a file served from a website. Peer-to-peer networks make use of the Internet to channel communications through specific ports, but a P2P network is not part of the World Wide Web. It “piggybacks” the Internet to connect netizens interested in trading files from their personal computers. Torrents are not illegal by definition, though illegal materials can be uploaded as torrents.

Torrent enthusiasts can upload original programs they’ve written, scripts, original musical compositions, or any other personal work for public consumption. Freeware can also be uploaded, and is legal to do so, as long as this does not violate the program’s Electronic User Licence Agreement (EULA). Typically, a freeware EULA stipulates that the software cannot be re-engineered before being redistributed, and that the EULA remain intact and bundled with the program.

Many uploaded torrents, however, are copyrighted materials shared without the owners’ authorization or knowledge. Controversy between P2P proponents and opponents center around whether or not file sharing between private citizens constitutes fair use under the law, or a breach of copyright protections.

Torrents first came to the public’s attention when Napster® was sued for its monstrously popular music-sharing P2P network. The original site, created by Shawn Fanning, operated from 1999-2001 before it was shut down by court order and later went bankrupt. The site acted as an index for the many P2P files available. Legendary rock band, Metallica and rapper Dr. Dre both sued Napster along with several recording labels. Fanning lost the case, sold Napster's assets, and Napster re-emerged under new ownership operating as a pay-for service.

In the case of music torrents, many people argue that they have already purchased the music previously in some other form (LP, cassette or CD) and simply want it in digital form without the hassle of ripping or transferring the music, or buying it twice. Others simply like getting music for free. Regardless, uploading copyrighted material for public distribution is illegal, unless the copyright holder has granted permission.

Other illegal torrents found on P2P networks include commercial movies. In some cases the movie file might be illegally obtained by using a handheld digicam to film the movie screen as the movie plays, resulting in a low quality facsimile. Once an official DVD release is available, the DVD image might be ripped and uploaded for sharing, which is also illegal.

Another category of illegal torrents consists of uploaded shareware that has been tampered with to bypass trial period restrictions. In many cases malicious hackers augment programs by injecting Trojans, viruses, rootkits or other malware into the software as a payload, then upload it to P2P networks for unsuspecting users to download and install. When the program is installed, so is the malware.

Unfortunately, the very nature of P2P networks perpetuates infected software and illegal torrents, as participants must upload at least as much as they download to maintain a 1:1 share ratio. Otherwise, their bandwidth or download speed is throttled back. This is a failsafe measure to ensure participants contribute to the network. Ergo, when downloading a torrent, the bits that are already downloaded are being uploaded simultaneously to other users who are also requesting that file. A user won't know if a file is infected until she or he has completely downloaded it and scanned it, and by then it has already been uploaded to several others. If the file is copyright protected, even if the user feels justified (legally or not) by having previously purchased the material, he or she will still be illegally re-distributing it to others in the download/upload process.

P2P networks are a highly successful, extremely popular way to exchange files between enthusiasts. However, users need to be aware that many available torrents are being illegally shared and many others are infected. A user's Internet Protocol (IP) addresses is also readily visible to all others on the P2P network, so anonymity is not an option.