You need a bittorrent client to download the files. There are many clients (built in Java, C, C++ etc). One of the more famous ones is Azureus (http://azureus.sourceforge.net)
Read this article
to know about bittorrent and various clients.
Once you download and install bittorrent client, you'll have to open up your .torrent file from within that BT client and it'll start downloading your ISO.