Torrent principle

The BitTorrent protocol was made mainly for temporarily very popular files - for example when a new movie or music album comes up. The download speed increases proportionately to the number of users downloading the same file and that is the biggest advantage of torrents.

When you are downloading something through torrent, you share your already downloaded part at the same time (for this reason you break the law in most countries when you are downloading illegal data). When you start downloading, your application contacts a "tracker" server, which co-ordinates all clients (other users' applications). From the tracker the application finds out where to download from, which parts have the other users already downloaded and so on. Also your application informs the tracker about what part you were able to download until now - and by that it gives it at other users' disposal.

Problems

A situation, when the tracker server is not functional may turn up. In this case you can't begin downloading, but if you already are downloading, it doesn't have to be necessarily a problem to finish the download (your client already knows where to download from).
Then there may be no one downloading or sharing but you. Therefore you have nowhere to download from. This happens with old and unpopular torrents.
Theoretically a situation when there is nobody that has the file complete may come up (such user is called "seed" or "seeder") and you simply can't get the whole file together from the parts other users have.

Continue: torrent clients