A small metadata file which contains information about the data you want to download, not the data itself. It is downloaded from a web site (BitTorrent file extension is .torrent) by clicking on a download link. It can also be saved to your computer. This is useful if you want to be able to re-open the torrent later on without having to find the link again.
A peer is another computer on the internet that is sharing the file you wish to download. Typically a peer does not have the complete file, if it did it would be called a seed. Peers are also called leeches, to distinguish them from those who have completed their download and continue to leave their BitTorrent Client running and act as a seed.
A computer that has a complete copy of the specific torrent you are downloading . Once your client finishes downloading, it will remain open until you click the Finish button. This is known as seeding. You can also start a BitTorrent Client with a complete file, and once BitTorrent has checked the file it will connect and seed the file to others. It is good to continue seeding a file after you have finished downloading, to help others finish. Also, when a new torrent is posted to a tracker, someone must seed it in order for it to be available to others. The tracker doesn't know anything of the actual contents of a file, so it's important to follow through and seed a file if you upload the torrent to a tracker.
When there are zero seeds for a given torrent (and not enough peers to have a distributed copy), all the peers will get stuck with an incomplete file, since no one in the swarm has the missing pieces. When this happens, someone with a complete file (a seed) must connect to the swarm so that those missing pieces can be transferred. This is called reseeding. Usually a request for a reseed comes with an implicit promise that the requester will leave his or her client open for some time period after finishing (to add longevity to the torrent).
In some versions of the client, you will see "Connected to x seeds; also seeing x.xxx distributed copies." A seed is a computer with the complete file. However, the swarm can collectively have a complete copy (or copies) of the file, even without seeds a complete distributed copy can be available if the sum total of the users have a complete copy among themselves. BitTorrent doesn't distribute in sequential order, so this is possible.
The group of users that are collectively connected for a particular file. Example, if you start a BitTorrent Client and it tells you that you're connected to 5 peers and 1 seeds, then the swarm consists of you and those 6 other people.
Server on the Internet that coordinates the action of BitTorrent Clients. Upon opening a torrent, you contact the tracker and receive a list of peers to connect to. Throughout the transfer, your computer will query the tracker, telling it how much you've downloaded and uploaded and how much before finishing. If a tracker is down and you try to open a torrent, you will be unable to connect. If a tracker goes down during a torrent (you have already connected at some point and are already talking to peers), you will be able to continue transferring with those peers, but no new peers will be able to contact you. Tracker errors are often temporary, leave the client open and continue trying.
A ratio of your amount uploaded divided by your amount downloaded. The amounts used are for the current session only, not over the history of the file. If you achieve a share ratio of 1.0, that would mean you've uploaded as much as you've downloaded. The higher the number, the more you have contributed. If you see a share ratio of this means infinity, which will happen if you open a BitTorrent Client with a complete file (i.e., you seed the file.) In this case you download nothing since you have the full file, and so anything you send will cause the ratio to reach infinity. While share ratings are just displayed for your convenience, courtesy to others should cause you to keep this ratio as high as possible.
When a connection is choked, it means that the transmitter isn't currently sending anything else on the link. A BitTorrent Client signals that it's choked to other clients for a number of reasons, the most common is that by default a client will only maintain --max_uploads active simultaneous uploads, the rest will be marked choked. A connection can also be choked for other reasons, for example a peer downloading from a seed will mark his connection as choked since the seed has no need to receive.
Periodically, the client shakes up the list of uploaders and tries sending on different connections that were previously choked, and choking the connections it was just using. You can observe this action every 10 or 20 seconds or so, by watching the "Advanced" panel of one of the experimental clients.
Term used in the protocol specification. refers to the state of a downloader with respect to a connection. A downloader is marked as interested if the other end of the link has any pieces that the client wants, otherwise the connection is marked as not interested.
If the client has not received anything after a certain period (default: 60 seconds), it marks a connection as snubbed, in that the peer on the other end has chosen not to send. See the definition of choked for reasons why an uploader might mark a connection as choked. The real function of keeping track of this variable is to improve download speeds. Occasionally the client will find itself in a state where even though it is connected to many peers, it is choked by all of them. The client uses the snubbed flag in an attempt to prevent this situation. It notes that a peer with whom it would like to trade pieces with has not sent anything in a while, and rather than leaving it up to the optimistic choking to eventually select that peer, it instead reserves one of its upload slots for sending to that peer.