bt junkie is a bit torrent tracker. A bit torrent tracker is where somebody uploads a torrent, most commonly of music, movies and software, for other people to download.
From the right sites you can get incredibly quick download speeds, as lots of people have the file and are uploading to you. The most popular bit torrent trackers (searchers) are:
the pirate bay
mininova
demonoid
isohunt
toorgle (like google, but for torrents)
uTorrent is a (and the best in my opinion) bit torrent client, which allows you download all the files in the torrent. the ".torrent" file you download simply tells utorrent (or any other client) about the files, etc.
Yes, you can put these items into itunes, if they are music or movie based. This is because they end up as .mp3, .avi, .mp4 or whatever format you downloaded them.
A word of warning
In order to download the files, people need to upload to you, so when you finish downloading you will be uploading. Unless restricted, which you are uploading you will be seriously slowing your internet connection, and may be unable to browse. A lot of data will also be transfered, so check if you have a monthly data limit. Also check if your internet provider has a fair usage policy, where you may not be able to download large amounts on a regular bases between specific times.