The Protocol:

BitTorrent is both the name of a peer-to-peer (P2P) file distribution client application and also the name of the file sharing protocol itself, both of which were created by programmer Bram Cohen. BitTorrent is designed to widely distribute large amounts of data without incurring the corresponding consumption in costly server and bandwidth resources. According to CacheLogic, BitTorrent traffic accounts for ~35% of all traffic on the internet.

The BitTorrent protocol breaks the file(s) down into smaller fragments, typically a quarter of a megabyte (256 KB) in size. Peers download missing fragments from each other and upload those that they already have to peers that request them. The protocol is 'smart' enough to choose the peer with the best network connections for the fragments that it's requesting. To increase the overall efficiency of the swarm (the ad-hoc P2P network temporarily created to distribute a particular file), the bittorrent clients request from their peers the fragments that are most rare; in other words, the fragments that are available on the least number of peers, making most fragments available widely across many machines and avoiding bottlenecks. The file fragments are not usually downloaded in sequential order and need to be reassembled by the receiving machine. It is important to note that clients start uploading fragments to their peers before the entire file is downloaded. Sharing by each peer therefore begins when the first complete segment is downloaded and can begin to be uploaded if another peer requests it. This scheme is particularly useful for trading large files such as videos and operating systems. This is different from conventional file serving where high demand can lead to saturation of the host's resources as the consumption of bandwidth to transfer the file to many requesting downloaders surges. With BitTorrent, high demand can actually increase throughput as more bandwidth and additional “seeds” of the file become available to the group. Cohen claims that for very popular files, BitTorrent can support about a thousand times as many downloads as HTTP.

The BitTorrent protocol operates on top of TCP.

Legal issues:

BitTorrent, like any other file transfer protocol, can be used to distribute files without the permission of the copyright holder. BitTorrent has received bad press (mostly initiated by incensed Hollywood movie distributors) for its ability to also illegally distribute copyrighted files. BitTorrent has also had unfounded accusations made against it that were later retracted after the BBC received many letters about it.

Proper Legal Use:

BitTorrent can be used by software developers who want to ease the bandwidth strain on their servers. If a developer offers a large file for download, the bandwidth limit of their server may be exceeded if a large number of people download the file. By offering the file via BitTorrent, they transfer much of the bandwidth burden to downloaders of the file.

For example, the site http://www.gameupdates.org offers legal game files via BitTorrent; the demo of the flight sim X-Plane is offered via BitTorrent, as are the World of Warcraft ingame patches. Another such example is PlaneShift, a free open-source MMORPG, which uses BitTorrent for its primary method of distribution. The fan-film Star Wars: Revelations is distributing two DVD images as well as the film by itself via BitTorrent, while Star Wreck: In the Pirkinning and Cactuses, both feature-length films, were provided for download via the network besides a centralized server. In 2005, the rock group Harvey Danger began distributing their third full-length album, Little by Little..., using BitTorrent. Also, various operating systems have used BitTorrent as an alternative way of distributing ISO images of their releases, including FreeBSD, NetBSD version 1.6.2 and later, and most major Linux distributions.

Peter Jackson's production diaries for King Kong have been posted for download using BitTorrent technology. Democracy Now!, a progressive news organisation, now distributes its daily television and radio broadcast using BitTorrent technology as well as by podcasting in addition to its traditional cable and satellite distribution. Several Anime companies have also used BitTorrent technology to release teaser episodes and trailers online for promotional purposes, as a sign of embracing technology that is often seen as a direct competitor. Furthermore, the NASA space agency recently included BitTorrent as a means to download some of their larger space image files.

Among the best legitimate sites for movies and music:

-- Legal Torrents ( http://www.legaltorrents.com/ ), which includes a wide selection of electronic music. It also has the Wired Magazine Creative Commons CD, which has songs from artists like the Beastie Boys who agreed to release some of their songs under a more permissive copyright that allows free distribution and remixing.

-- Torrentocracy ( http://torrentocracy.com/torrents/ ) has videos of the U.S. presidential debates and other political materials.

-- File Soup ( http://www.filesoup.com ) offers open-source software and freeware, music from artists whose labels don't belong to the Recording Industry Association of America trade group, and programs from public television stations like PBS or the BBC.

-- Etree ( http://bt.etree.org ) is for devotees of "trade-friendly" bands like Phish and the Dead, who encourage fans to share live recordings, usually in the form of large files that have been minimally compressed to maintain sound quality.

Information sources: wikipedia, various web pages, & a little from me.

Official BitTorrent Home Page: www.bittorrent.com
Wikipedia BitTorrent Page: http://en.wikipedia.org/wiki/Bittorrent

Well I don't know what this topic is doing here to tell you the truth and I'm sort of confused now.

I guess this is one of those "nice to know" -topics. Who knows, this might actually be of use to someone. And it's never a bad thing to have a general idea of all that technical stuff.

How the downloads work:

BitTorrent uses .torrent files to download the desired content. The .torrent files contains the data on the files, the “tracker” url(s) and the hash. A centralized server called a “tracker” is used as an IP list of everyone sharing a .torrent file. Your BitTorrent client connects to the tracker to obtain this IP list and uses it to connect to other peers using the same .torrent as you. When you contact the tracker your IP is added to the list and other users will starter connecting to you as well. Your speed will depend on many thins: your connection, your client settings, and the Seed vs. Leech ratio. Most important, BitTorrent works tit for tat with a ratio. The higher ratio you have on a .torrent the more you get and sooner, if your ratio is low you have to wait longer and get lower speeds.

Seeds or “seeders” are people who have a completed copy of the data being shared, and are hosting these files for upload to the swarm. It’s common courtesy to give back at least the same amount you downloaded. Leechs or “leechers” are people who have not completed the copy and are currently uploading. The swarm is the group of people seeding and leeching a .torrent file.

Torrent files are very secure because if they are edited the hash will change and the edited .torrent will not work with the unedited versions.

Features to look for in a client:

DHT (Distributed Hash Table) DHT allows people to continue sharing the data from a .torrent file if the tracker is down for whatever reason. There are currently two kinds of DHT, Azureus DHT & Mainline DHT. At this point in time most clients only use Mainline DHT while only Azureus has Azureus DHT.
PEX (Peer Exchange) PEX allows clients to trade their IP lists between each other. Currently there are two kinds of PEX regular PEX and an updated version by µTorrent. At this point in time only µTorrent uses the new PEX and Azureus should be switching over soon.
PE/MSE (Protocol Encryption / Message Stream Encryption) This feature is relatively new. It encrypts communication between peers on a .torrent. If gives added security and helps prevent ISPs from interfering with your speeds. This was developed between µTorrent & Azureus. The only other client to support this is BitComet.

Clients:

Mainline this is the official BitTorrent client. It’s ugly and lacking in features. However it is coded in Python so it will work in Windows, Mac, & Linux.

µTorrent (pronounced “you-torrent”) written in C++ using custom Libraries. This client is for Windows only. It only uses 6-10 megabytes of memory and low CPU usage yet remains feature rich.

Azureus coded in Java. Azureus will work on any OS that is capable of running Java. This client is very feature rich, customizable, and open source. Windows users should beware the resource usage of Java.

BitComet written in C++. BitComet is the black sheep of the BT community. BitComet is a strong feature rich client that is very customizable, too customizable. BitComet can easily be used to cheat downloads. Thus it has been banned from most private sites.

BitTornado is a python recode of Mainline. BitTornado is very basic yet still an effective BitTorrent client.

I just wanted to tell people about this great download tool.

BitTorrent is perfectly legal to use. Now what you're shareing is a different story, but I posted links for that above.