Well BitTorrent is a program used to send big files pretty easily without huge bandwidth issues. The program itself is legal, what you choose to download using it may be illegal. For instance you can use BitTorrent to download a patch for a video game you own which would be 100% legal. But, if you use BitTorrent to download a copyrighted music or video file that would be illegal. You can get caught by downloading illegal files, I'm not just saying that to scare you but, yes, I did get a letter from my ISP with a list of a game I downloaded, it was a dumb thing to do and I learned my lesson.
Think before you do something, if it's something as silly as downloading a CD, just go to the store or online and buy it, it's not worth the risk of getting tangled up with the RIAA, MPAA, etc...
Summary: BitTorrent is legal, but the files you download from it can be illegal. Just be smart about it and you'll be fine.