BitTorrent Terminology

torrent
A torrent can mean either a .torrent metadata file or all files described by it, depending on context. The torrent file contains metadata about all the files it makes downloadable, including their names and sizes and checksums of all pieces in the torrent. It also contains the address of a tracker that coordinates communication between the peers in the swarm.

swarm
Together, all users sharing a torrent are called a swarm. Six peers and two seeds make a swarm of eight.

peer
A peer is one instance of a BitTorrent client running on a computer on the Internet that you connect to and transfer data. Usually a peer does not have the complete file, but only parts of it, however, 'peer' can be used to refer to any participant in the swarm (in this case, also known as a 'client').

seed
A seed is a peer that has a complete copy of the torrent and still offers it for upload. The more seeds there are, the better the chances are for completion of the file.

leech
A leech is usually a peer who has a negative effect on the swarm by having a very poor share ratio - in other words, downloading much more than they upload. Most leeches are users on asynchronous internet connections who through either ignorance or choice do not leave their BitTorrent client open to seed the file after their download has completed. However, some leeches intentionally hurt the swarm to avoid uploading by using modified clients or excessively limiting their upload speed.
The term leech is also incorrectly used to refer to what should properly be called a peer, a member of the swarm who has not yet downloaded the complete file.

tracker
A tracker is a server that keeps track of which seeds and peers are in the swarm. Clients report information to the tracker periodically and in exchange receive information about other clients that they can connect to. The tracker is not directly involved in the data transfer and does not have a copy of the file.

availability
(also distributed copies) The number of full copies of the file available to the client. Each seed adds 1.0 to this number, as they have one complete copy of the file. A connected peer with a fraction of the file available adds that fraction to the availability (ie. a peer with 65.3% of the file downloaded increases the availability by 0.653).

interested
Describes a downloader who wishes to obtain pieces of a file the client has. For example, the uploading client would flag a downloading client as 'interested' if that client did not possess a piece that it did, and wished to obtain it.

choked
Describes an uploader to whom the client does not wish to upload. An uploading client 'chokes' another client in several situations:
The second client is a seed, in which case it does not want any pieces (ie. it is completely uninterested)
The uploading client is already uploading at its full capacity (ie. the value for max_uploads has been reached)

snubbed
An uploading client is flagged as snubbed if the downloading client has not received any data from it in over 60 seconds.

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