Um, yeah, where to begin... It's the "new" style of file sharing where everyone connects to a "tracker" to find "peers" that are trying to share the same file(s), and share it directly with peers they connect to. I'm sure Wikipedia can do a better job of explaining.
Let's just say that there are certain torrent sites that have loads and loads of valuable data. Some require you to upload a certain ratio of your DL to enforce the core of BT, which is that torrents only work if there are people to seed (provide the complete files).