The client is the P2p(peer to peer) program you use to download the torrent which is the file. Here is a list of all clients
http://en.wikipedia.org/wiki/Comparison_of_BitTorrent_software and what they support. For instance you use Azureus (the p2p program) to download the newest torrent file of Planeshift. The list on the page also give links to each repected client site to get the program. Hope this clears up some of the confusion. It seems a lot of people are confused about torrents and clients and which is which.
