To answer your question about what is a torrent:
A torrent is a way to minimize bandwitdth and the cost of sharing files on the Internet.
Instead of transmitting the same file to several users, you transmit a part of the file to one user and another part to another user. The first user retransmit what he has already received to the second and the second retransmit the part he has received to the first.
You can "fragment" the file and send it to many users once (ideally) and they retransmit between them the fragments they've already received.
Imagine you have to make photocopies of a book: instead of photocopying the entire book many times, you can photocopy it once, send a part to a person, another part to another person and so one. Then, the people who got the copies, reproduce the fragments they've received and share them until everybody has a complete book.
The people transmitting the "original file" are called seeders. The ones retransmitting the parts they've already received are called leechers. If the torrent "runs out" of seeders, it's dead.
So: download any of the software already suggested by other members.
Look for a torrent that has enough "seeders". And enjoy.
The transmision is slower than "normal": you have to receive the file from many computers on the Net and, besides, your computer retransmit parts of it to other users.
Probably the most popular source of torrents is "The Pirate Bay", which has fought an ardous legal battle, just look at their name. They only host "tracker" servers that keep information about how the file is being distributed, who are the "seeders" and who are the "leechers". The whole process is transparent to you: you simply pick a file and wait for the computer to download it.
After you finish your download, it's common courtesy to left your computer connected for a while: it will become a "seeder" and keep the torrent "alive". If nobody does that, the torrent runs out of "seeders" and it is possible that nobody connected to the torrent has a complete copy of the file:
in this case, your download will stop at some percentage of the download (usually 99%... 
)
If you have Opera (have I already recommended it?) as your browser, you don't need external software: it's integrated.
