read
http://www.bittorrent.com/introduction.html to get an idea how you download files in bittorrent.
if you don't get it, it's basically like this.
Seeds are people who have finished downloading the file and upload at their highest (or limited) speed. They don't download anything anymore because they're finished with the file (unless they are downloading with other programs or other bittorrent files).
Leechersa are people who are currently downloading the file.
you will download parts of the file from both seeders and leechers. But if there is no seeder and only one leecher at 90% done, then you can only download to 90% (provided that the leecher stays on for you to download off him). If there are no seeders and no leechers, or the tracker (where the bittorrent file is kept) then you can't get the file.
You might want to try
Bitcomet because it's written in C++ and doesn't slow down your computer like many other bittorrent clients do. It'll take a bit to get used to, but in the end you'll get it.
dartworth, you beat me right to it. i wasted a minute of my life typing all that