What's a "torrent"?

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Summary: BitTorrent is a peer-to-peer file sharing technology.

Torrents, typically ending in ".torrent", are control files for the peer-to-peer file sharing technology called BitTorrent.

It's actually pretty cool technology that really just boils down to another way to download files. So really, you just use .torrent files to download other files.

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BitTorrent is a distributed file distribution technology. Yes, "distributed distribution". When you download a file using BitTorrent, the file is actually broken up into chunk that your BitTorrent client program then downloads and reassembles into the final file as the pieces arrive. To over-simplify, what makes it interesting are two things:

• The different chunks you download can all be coming from different machines. A BitTorrent client will connect to many other BitTorrent clients and download several chunks at once, in random order. In the long run this makes the protocol fairly efficient, and very nicely scalable - the more BitTorrent clients that are serving up a given file, the faster other clients can download it.

• As you start collecting chunks of the file, your BitTorrent client will start making those chunks available for downloading to other BitTorrent clients, and will become a part of the peer-to-peer file distribution network.

The ".torrent" file is simply the bootstrap for this whole process. You download that normally, for example in your web browser, and then it is read by your BitTorrent client. It has the information that the BitTorrent client then uses to begin to locate other BitTorrent clients that are serving up the file you're interested in.

"Torrent" is sometimes used to refer to the file being shared using BitTorrent, but a ".torrent" file is a specific file with specific information that is used to bootstrap the file download.

Note I haven't talked at all about what kinds of files are actually being downloaded. That's because the answer is "any". Typically the types of files being shared using BitTorrent are large - audio files, video files and programs. But just saying "a torrent" doesn't tell you what it is or what you can do with it.

Let's get concrete. Let's say you've discovered that some "Public Domain Movie" is available via a torrent. You download the ".torrent" file, and open it in your BitTorrent client. It then goes out to the internet, locates other BitTorrent clients that are serving up that file, and begins downloading all the various chunks until it has a complete copy of the file. When it's all done, you'll end up with something like a "PublicDomainMovie.avi" file. Or ".mpg", ".mov" or something else. What you then do with that file is up to you. Yep, you could burn that file to a CD or DVD, but that's all something you would do after it's downloaded, and has nothing to do with the fact that you got it via BitTorrent.

I don't have a tremendous amount of experience with BitTorrent clients, so I can't really recommend one over the other, but I have used Azureus successfully, and it seems to be one of the more popular BitTorrent clients. I will warn you that all of the BitTorrent clients I've seen to date are still in the "geeky" stage, meaning that they sort of assume you know what you're doing to begin with. It's not a steep learning curve, but it will seem pretty obscure at first.

Related:

• Just how long should a download take? How quickly a download takes depends on many things including the size of the file and the speed of your connection, among other things.

• Can I check a download for viruses before I download it? Files cannot be checked for viruses before being downloaded.

Article C2564 - February 20, 2006