I'm going to come right out and say it, BitTorrent is a tool of massive copyright infringement.
But so was the VCR, and the cassette recorder. Even the Kodak Brownie, the first popular consumer camera, was ridiculed for copyright infringement, as well as privacy concerns.
But the reason the above technologies survived, and flourished, was not due to their copyright infringing abilities. It was due to their usefulness within the law.
BitTorrent is extremely useful, and more so every day. It is a decentralized way to store and transmit large files. A user downloads a small seed file that identifies the target file. The user then runs an open source BitTorrent client, which searches across the Internet for other peers, other users operating the client and possessing the seed file. As peers are identified, the target file is copied and transferred to the user from each peer. But the whole target file is not transferred from any one user to another. The large file is broken up into tiny chunks, or bits. This way, a small portion of the file is transferred from each peer, until all the bits comprising the entire target file are downloaded by the user.
Upon first glance, this may seem like an overly complicated way to transfer files. How could this possibly be useful?
Here are two reasons. The first is that the traffic load is distributed across peers. Since the file is coming from multiple peers across the globe, it is only relying on a relatively small amount of data from each peer. This lessens the burden on each user's system and internet connection. Additionally, since the load is distributed, if one of the peers goes off line, or the internet connection is lost, those bits can come from other peers. Once the target file is downloaded, the user may become a peer for others to acquire the file.
This is what the Internet was designed to do. The original application of the Internet was a command and control system created by the Defense Advanced Research Projects Agency (DARPA) for the US Department of Defense. It was intended to be the controls for launching nuclear weapons, and a safe guard against nuclear attack. If one of the command centers was hit by a bomb, the others would maintain their connection to each other and the weapons.
This is how the internet works for most communication. Each transfer of data, in the form of a packet of bits, is sent through one of several routes in the network. The web page you are looking at now may have come through dozens of different routes to get to your browser, all originating from the data center in San Francisco where this site is hosted. The difference is that BitTorrent files are coming from multiple hosting sites, adding another dimension of reliability to the transmission.
The second reason BitTorrent is useful is because it delivers high quality content. When a user selects a seed file, the number of peers is often estimated. By selecting a seed file with a larger number of peers, a user can be more confident of the quality of the file. Peers are virtually anonymous. It is possible that some peers would share corrupted files, on accident or on purpose, but it is much less likely that a large number of peers would. If a user downloads a file through BitTorrent and discovers its corrupt or inferior, the likely reaction is to delete the file, not to share it with others. When there are a large number of peers seeding the same file, the user knows that multiple anonymous people have all downloaded and found that file useful. This is distributed anonymous quality control.
Technology is always ahead of the law, and plaintiffs have not consistently and successfully prosecuted copyright infringers using BitTorrent, but there are several examples out there. The predominant legal tactic is to gather IP addresses from peers seeding a copyright protected file, subpoena the Internet providers associated with the IP addresses for customer names, and then sue the individuals. Some defendants have successfully removed the cases on various grounds, but may of them settle out of court for a few thousand dollars.
I do not condone the use of BitTorrent for illegal purposes. Similarly, I do not condone the use of any other legal technology for illegal purposes. Don't speed in your car either.
But I do encourage you to check out BitTorrent, learn what it is about and use it for legal purposes. What are the available legal purposes? The transfer of large files by the copyright holder, or in the public domain. Click this link and this link for information about legal BitTorrent sites.