1. A torrent is a file you can open. It contains information needed for a compatible software to get a download over the Internet from other BitTorrent clients.
2. Yes you can, but this is not recommended, as it won't give you more performance in most cases (if 2 of these programs are using the same network). if you wish to get access to more networks, use a multi-network client. (For example: Shareaza)
3. No. All good programs are completely free. But using them might be illegal under certain circumstances in your country.

More information:
BitTorrent - Wikipedia, the free encyclopedia
Shareaza - Wikipedia, the free encyclopedia

Greetings,
Old

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</td><td><a href="http://crawler.trillinux.org"><img align="right" style="border:0" src="http://crawler.trillinux.org/img/netsize.png" alt="Network size"></a></td></table>

Thanks the info made things more clear.