Well Bitorrent (& other variants) isn't always used for warez. It has many legitimate uses such as downloading Linux distributions, movie trailers, game demos, etc.
In short, to download a file from the Bitorrent network, you need to find torrent files which will help you locate "seeders" (people sharing the file). Once you have that torrent, it's only a matter of opening it with utorrent & letting it download the file. Common courtesy would ask that once you've finshed downloading the file, you leave it shared & play the role of a seeder yourself for a while.