by Robbie » Aug 19th, '11, 14:47
The technical difference between a torrent and a straight download is that a torrent is a file that's broken up into fragments. When you download a torrent, you receive it as a flood of fragments from many different people (hence "torrent").
The best analogy I've seen is comparing a torrent to a large book. Suppose a book has been chopped up into individual pages. If you want the book, you start by finding somebody who has a page you need and asking them to give you a photocopy of that page. Some people might have lots of pages and be willing to give you free copies of all of them. Other people might only give you a page if you're willing to trade a page that you have and they need. Assuming at least one person has the whole book (so all the pages are in existence somewhere), then eventually everybody in the "pool" will end up with a copy of it.
Since you're gathering up your book page by page and trading pages with other people, it follows that you're giving back something to the "pool" even if all you want to do is get your own copy. This is the nature of a torrent -- you're downloading what you need and at the same time uploading the bits you already have that other people need.
If you stay in the torrent after you've gathered the whole file, you become a "seed" or "seeder" who makes it possible for other people to collect all the pieces they need. People who haven't yet gathered the whole file are "peers" or "leeches" -- although "leech" is a bit unfair, since they're necessarily contributing while they're in the torrent.
There are plenty of legal torrents. For instance, I'm seeding a promotional Esperanto DVD which was made to be copied and distributed freely. Project Gutenberg makes its compilation disks available in torrent form. It's easier for everybody when it comes to big files like DVDs, because a torrent can be stopped and started, whereas a download has to be done all at once or it's ruined.
But of course, most torrents are illegal copies of movies, video games, books, etc. Because of this, and since you have no idea who created the files, there's always a risk of viruses, especially on public torrent sites.
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