Using a web browser, users navigate to the site listing the torrent, download it, and open it in a BitTorrent client. After opening the torrent, the BitTorrent client connects to the tracker, which provides it with a list of clients currently downloading the file or files. A group of peers on a BitTorrent or P2P connected with each other to share a particular torrent is generally referred to as a swarm.

Initially, there may be no other peers in the swarm, in which case the client connects directly to the initial seeder and begins to request fragments. The BitTorrent protocol breaks down files into a number of small fragments, typically a quarter of a megabyte (256 KB) in size. Larger file sizes typically have larger fragments. For example, a 4.37 GB file will often have a fragment size of up to 4 MB (4096 KB). File fragments are checked as they are received using a hash algorithm to ensure that they are error free.

As peers enter the swarm, they begin sharing fragments with one another. Because clients share fragments with one another, instead of directly from the seeder, BitTorrent networks easily scale to large numbers of clients. The protocol incorporates mechanisms so that clients choose peers with the best network connections for the fragment they are requesting. One major innovation that adds to the scalability of BitTorrent is the concept of “rare fragments.� The BitTorrent protocol specifies that clients should always request fragments that are the rarest, meaning they are held by the fewest number of clients in the swarm. By requesting the rarest fragments, the BitTorrent protocol ensures that one machine will not be swamped with requests, eliminating potential network bottlenecks.

BitTorrent also uses a mechanism called “optimistic unchoking� to create a group of “preferred peers.� Preferred peers are a subset of peers in a swarm who are known to offer the most bandwidth. Optimistic unchoking allows BitTorrent to determine which peers should be preferred by periodically testing peers. Peers outside the group are offered a file fragment. If the peer downloads the fragment faster than a peer in the preferred peered group, the new peer displaces the old. This ensures that clients are always downloading from a group of the fastest available peers.

From Wikipedia

BitTorrent does not offer its users anonymity. Because trackers maintain lists of file sharers, it is possible to obtain the IP address of all current, and possibly previous, file sharers. It is also possible to ascertain the address of peers in the current swarm, thus making peers open to attack by hostile parties.

Another drawback to the BitTorrent model is that file sharers have little incentive to become seeders after they finish downloading. The result of this is that seeds gradually die out as the swarm surrounding them dies out, meaning a lower possibility of obtaining older torrents. This has been addressed by certain tracking websites that record each user download/upload ratio and provide access to older torrent files to people with better ratios.

BitTorrent excels in continuously connected, broadband environments. Dial-up users find it less efficient due to frequent disconnects and slow download rates.

From Wikipedia

Azureus implements the BitTorrent protocol using java language and comes bundled with many invaluable features for both beginners and advanced users:

• Multiple torrent downloads
• Upload and download speed limiting, both globally and per torrent
• Advanced seeding rules
• Adjustable disk cache
• Only uses one port for all the torrents
• UPnP sets the forward on your router
• Can use a proxy, for both tracker and peer communications
• Fast resume
• Can set a default download dir and move completed files
• Can import torrents automatically from a set dir
• Highly customizable interface
• IRC plugin included for quick help
• Embedded tracker, host your own torrents, automate your shares (periodic scanning of a dir)...
• Distributed Hash Table: host on it a completely decentralised torrent and/or benefit from it when the tracker is offline

Many useful plugins are available:

• Auto-adjustment of your upload and download speeds according to your network charge with Autospeed
• Auto-download torrents with RSS Feed Scanner
• Schedule upload and download speed limits with Speed Scheduler
• Receive a mail when a download finishes with Status Mailer
• Add numerous info on torrents (Extra seeding columns, Country Locator)
• Remote control Azureus through the Swing Web Interface or the HTML Web Interface

From the Azureus Official Page

What is BitComet (A powerful C++ BitTorrent Client)?

BitComet is a p2p file-sharing freeware fully compatible with Bittorrent, which is one of the most popular p2p protocol designed for high-speed distribution of 100MB or GB sized files. BitComet is a powerful, clean, fast, and easy-to-use bittorrent client. It supports simultaneous downloads, download queue, selected downloads in torrent package, fast-resume, chatting, disk cache, speed limits, port mapping, proxy, ip-filter, etc.

Features

• Support preview of video file which is being downloaded.
• Support DHT Network, download trackerless torrent!
• Completely new core written in C++, stable and fast, very low CPU usage.
• Ability to limit the upload speed as well as download speed.
• Clean and free, without any adware or spyware. ( Please note: Spybot may misreport "Comet Cursor" )
• Multiple simultaneous downloads, ability to select download files in one torrent and set file priority.)
• Intelligent Connection Optimize, Auto Optimization for different connections, runs well using all default settings.
• Intelligent Rate Control, optimize the upload distribution in order to get the max download rate.
• Intelligent Disk Cache, decrease the potential damage to the hard disk when high-speed downloading (>500KB/s) by decrease the read / write frequency, extra memory usage is the cost.
• Intelligent Disk Allocating, no long-time disk allocation, also decrease the disk fragment to almost zero.
• Intelligent Hash Scan, no time-consuming scanning when seeding / resume.
• Firewall and NAT Traversal, users behind differect NAT can now connect to each other
• Auto config ICF (Internet Connection Firewall) and ICS (Internet Connection Sharing) in Windows XP
• Auto config port mapping in router (UPnP support is required in router, Windows XP is required).
• Ability to block IP temporarily or permanently, compatiable with eMule ipfilter.dat.
• Ability to chat with other downloaders ( disabled by default )
• Compatiable with Windows XP SP2 TCP/IP restriction
• ZIP package is also provided, which runs withoust installation.
• Using only one TCP listening port.
• Support Multi-tracker, and utf-8 extension, UDP tracker Protocol v2.
• [Please Note]: It is reported that Zone Alarm 5.0 conflict with BitComet (and other p2p maybe).
• Support automatically power off with conditions.

From the BitComet Official Page

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

BitTorrent is trademark of BitTorrent Inc and its use is subject to a trademark use policy.

The original BitTorrent application was written in Python. Its source code, as of version 4.0, has been released under the BitTorrent Open Source License (a modified version of the Jabber Open Source License). There are numerous compatible clients, written in a variety of languages and running on a variety of computing platforms.

BitTorrent clients are programs which implement the BitTorrent protocol. Each BitTorrent client is capable of preparing, requesting, and transmitting any type of computer file over a network using the BitTorrent protocol. This includes text, audio, video, encrypted content, and other types of digital information.

From Wikipedia

Turbo Torrent is a P2P file-sharing freeware fully compatible with Bittorrent, which is one of the most popular P2P protocol designed for high-speed distribution of large files (i.e., 100MB or GB sized files).

Turbo Torrent is a powerful, clean, fast, and easy-to-use Bittorrent client. It supports simultaneous downloads, download queue, selected downloads in torrent package, fast-resume, chatting, disk cache, speed limits, port mapping, proxy, ip-filter and more.

Turbo Torrent is also the most visually appealing as well as feature rich Bittorrent client. It aims to provide a gross amount of statistical and network data in a visually empowered format.

Turbo Torrent is the only Bittorrent client that comes complete with a built-in Web based Torrent Search Engine, so you can search for files without the need to rummage around countless forums and Web sites.

Turbo Torrent is not advertising supported, nor does it require registration, and no information is collected from or about product users. It is very small and easy to use and comes complete with full install and uninstall wizard.

From the TURBOTORRENT official page

TorrentFlux is a FREE PHP based Torrent client that runs on a web server. Manage all of your Torrent downloads through a convenient web interface from anywhere.

TorrentFlux is a feature rich Torrent manager.

• Upload Torrents via URL or File Upload
• Start, Stop, and Delete Torrents with a click
• Advanced Torrent start options (ports, speeds, etc.)
• Multi-user interface
• RSS Feeds, download Torrents files with a click
• View Download Progress of all torrents at a glance
• View drive space at a glance
• View Torrent file meta information
• Built-in User management and Security
• Private Messaging
• Themes (selectable per user)
• Upload History
• Detailed User Administration
• Admin Searchable Logs
• Advanced Torrent Search (many major sites included)
• Cookie support at the user level for private trackers
• Queue Manager
• Maketorrent -- supports making torrents
• Language Support

From the TORRENTFLUX official page

A powerful C++ BitTorrent Client

BitLord is a p2p file-sharing freeware program fully compatible with Bittorrent, which is the most popular p2p protocol designed for high-speed distribution of 100MB or GB sized files. BitLord is a powerful, clean, fast, and easy-to-use bittorrent client. It supports simultaneous downloads, download queue, selected downloads in torrent package, fast-resume, chatting, disk cache, speed limits, port mapping, proxy, ip-filter, etc.

From the BITLORD official page