BitTorrent

• The BitTorrent protocol is a decentralized file transfer protocol. Unlike serial file transfer, where each user connects to the source and downloads the entire file, BitTorrent turns every downloader into an uploader as well. Once you receive a piece of the target file, your computer begins offering that piece to other downloaders, adding your upstream bandwidth to the total bandwidth available in the swarm. This allows files to propagate quickly as more users join the download, instead of forcing every downloader to share the upstream capacity of a single source.
  
  

Trackers

• When you click on a .torrent file, uTorrent reads the file information as well as the address of a central tracking server. uTorrent contacts the tracker, reports that it has no pieces of the file yet, and receives a list of users' addresses that have some part of the target. The client then contacts these users, requesting pieces, and begins to download the file. As each piece comes in, uTorrent slowly begins to assemble the target file while requesting new connections for subsequent pieces.
  
  

DHT

• UTorrent also features the option of using Distributed Hash Tables to further decentralize file transfers. Previously, .torrent files would have to come from a single source and utilized a single tracker, making them vulnerable to disruption. With DHT, uTorrent takes the cryptographic hash signature of the file and searches online users for others transferring that same file. If it finds any other active transfers, it requests the file from them as well. Users often share hash signatures in so-called "magnet links," allowing them to initiate BitTorrent transfers without contacting a specific tracker.
  
  

Seeding

• Once you have downloaded the complete file, uTorrent remains online as part of the swarm, uploading pieces to other users. At this point, you are a "seeder," or a complete source for the target file. BitTorrent users consider it a common courtesy to leave your client running after completion, to allow others to download pieces of the file they need. UTorrent tracks participation with a ratio of data downloaded to data uploaded, and in some file sharing communities, disconnecting before that ratio reaches a designated level may violate the rules.